[One (M7) Q&A, Help & Troubleshooting] nkk71:
ruu.zip method doesn't need a separate hboot downgrade first, it get's automatically downgraded during the first "fastboot flash zip ruu.zip", then the phone will reboot back to RUU mode, and you can issue the second "fastboot flash zip ruu.zip" which will install the rest of the ROM, firmware etc.

This is what it's gonna look like:

C:\ADB3>[B][COLOR="Blue"]fastboot oem rebootRUU[/COLOR][/B]
...
(bootloader) Start Verify: 0
OKAY [ 0.047s]
finished. total time: 0.047s

and this is what fastboot ruu mode looks like:

C:\ADB3>[B][COLOR="Blue"]fastboot flash zip ruu.zip[/COLOR][/B]
sending 'zip' (1033216 KB)...
OKAY [ 41.559s]
writing 'zip'...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) start image[hboot] unzipping for pre-update check...
(bootloader) start image[hboot] flushing...
(bootloader) [RUU]WP,hboot,0
(bootloader) [RUU]WP,hboot,99
(bootloader) [RUU]WP,hboot,100
(bootloader) ...... Successful
[B][COLOR="Red"]FAILED (remote: 90 hboot pre-update! please flush image again immediately)[/COLOR][/B]
finished. total time: 43.118s

^^ as expected "flush again"

C:\ADB3>[B][COLOR="Blue"]fastboot flash zip ruu.zip[/COLOR][/B]
< waiting for device >
sending 'zip' (1033216 KB)...
OKAY [ 41.912s]
writing 'zip'...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) start image[sbl3] unzipping & flushing...
(bootloader) [RUU]UZ,sbl3,0
(bootloader) [RUU]UZ,sbl3,100
(bootloader) [RUU]WP,sbl3,0
(bootloader) [RUU]WP,sbl3,100
(bootloader) ...... Successful
(bootloader) start image[boot] unzipping & flushing...
(bootloader) [RUU]UZ,boot,0
(bootloader) [RUU]UZ,boot,15
(bootloader) [RUU]UZ,boot,27
(bootloader) [RUU]UZ,boot,41
(bootloader) [RUU]UZ,boot,54
(bootloader) [RUU]UZ,boot,68
(bootloader) [RUU]UZ,boot,81
(bootloader) [RUU]UZ,boot,95
(bootloader) [RUU]UZ,boot,100
(bootloader) [RUU]WP,boot,0
(bootloader) [RUU]WP,boot,99
(bootloader) [RUU]WP,boot,100
(bootloader) ...... Successful
(bootloader) start image[rpm] unzipping & flushing...
(bootloader) [RUU]UZ,rpm,0
(bootloader) [RUU]UZ,rpm,100
(bootloader) [RUU]WP,rpm,0
(bootloader) [RUU]WP,rpm,100
(bootloader) ...... Successful
(bootloader) start image[pg2fs_spcustom] unzipping & flushing...
(bootloader) [RUU]UZ,pg2fs_spcustom,0
(bootloader) [RUU]UZ,pg2fs_spcustom,38
(bootloader) [RUU]UZ,pg2fs_spcustom,99
(bootloader) [RUU]UZ,pg2fs_spcustom,100
(bootloader) ...... Successful
(bootloader) start image[tz] unzipping & flushing...
(bootloader) [RUU]UZ,tz,0
(bootloader) [RUU]UZ,tz,100
(bootloader) [RUU]WP,tz,0
(bootloader) [RUU]WP,tz,100
(bootloader) ...... Successful
(bootloader) start image[sbl1-2] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-2,0
(bootloader) [RUU]UZ,sbl1-2,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl1-1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-1,0
(bootloader) [RUU]UZ,sbl1-1,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[adsp] unzipping & flushing...
(bootloader) [RUU]UZ,adsp,0
(bootloader) [RUU]UZ,adsp,16
(bootloader) [RUU]UZ,adsp,26
(bootloader) [RUU]UZ,adsp,37
(bootloader) [RUU]UZ,adsp,49
(bootloader) [RUU]UZ,adsp,60
(bootloader) [RUU]UZ,adsp,71
(bootloader) [RUU]UZ,adsp,83
(bootloader) [RUU]UZ,adsp,94
(bootloader) [RUU]UZ,adsp,100
(bootloader) [RUU]WP,adsp,0
(bootloader) [RUU]WP,adsp,100
(bootloader) ...... Successful
(bootloader) start image[dzdata] unzipping & flushing...
(bootloader) [RUU]UZ,dzdata,0
(bootloader) [RUU]UZ,dzdata,5
(bootloader) [RUU]UZ,dzdata,14
(bootloader) [RUU]UZ,dzdata,19
(bootloader) [RUU]UZ,dzdata,29
(bootloader) [RUU]UZ,dzdata,38
(bootloader) [RUU]UZ,dzdata,47
(bootloader) [RUU]UZ,dzdata,52
(bootloader) [RUU]UZ,dzdata,57
(bootloader) [RUU]UZ,dzdata,66
(bootloader) [RUU]UZ,dzdata,71
(bootloader) [RUU]UZ,dzdata,76
(bootloader) [RUU]UZ,dzdata,81
(bootloader) [RUU]UZ,dzdata,90
(bootloader) [RUU]UZ,dzdata,97
(bootloader) [RUU]UZ,dzdata,100
(bootloader) [RUU]WP,dzdata,0
(bootloader) [RUU]WP,dzdata,11
(bootloader) [RUU]WP,dzdata,23
(bootloader) [RUU]WP,dzdata,36
(bootloader) [RUU]WP,dzdata,70
(bootloader) [RUU]WP,dzdata,77
(bootloader) [RUU]WP,dzdata,98
(bootloader) [RUU]WP,dzdata,100
(bootloader) ...... Successful
(bootloader) start image[tp] unzipping & flushing...
(bootloader) [RUU]UZ,tp,0
(bootloader) [RUU]UZ,tp,100
(bootloader) ...... Successful
(bootloader) start image[sbl2] unzipping & flushing...
(bootloader) [RUU]UZ,sbl2,0
(bootloader) [RUU]UZ,sbl2,100
(bootloader) [RUU]WP,sbl2,0
(bootloader) [RUU]WP,sbl2,100
(bootloader) ...... Successful
(bootloader) start image[recovery] unzipping & flushing...
(bootloader) [RUU]UZ,recovery,0
(bootloader) [RUU]UZ,recovery,11
(bootloader) [RUU]UZ,recovery,21
(bootloader) [RUU]UZ,recovery,32
(bootloader) [RUU]UZ,recovery,44
(bootloader) [RUU]UZ,recovery,72
(bootloader) [RUU]UZ,recovery,85
(bootloader) [RUU]UZ,recovery,96
(bootloader) [RUU]UZ,recovery,100
(bootloader) [RUU]WP,recovery,0
(bootloader) [RUU]WP,recovery,99
(bootloader) [RUU]WP,recovery,100
(bootloader) ...... Successful
(bootloader) start image[system] unzipping & flushing...
(bootloader) [RUU]UZ,system,0
(bootloader) [RUU]UZ,system,1
(bootloader) [RUU]UZ,system,2
(bootloader) [RUU]UZ,system,3
(bootloader) [RUU]UZ,system,4
(bootloader) [RUU]UZ,system,5
(bootloader) [RUU]UZ,system,6
(bootloader) [RUU]UZ,system,7
(bootloader) [RUU]UZ,system,8
(bootloader) [RUU]UZ,system,8
(bootloader) [RUU]UZ,system,9
(bootloader) [RUU]UZ,system,10
(bootloader) [RUU]UZ,system,11
(bootloader) [RUU]UZ,system,12
(bootloader) [RUU]UZ,system,14
(bootloader) [RUU]UZ,system,15
(bootloader) [RUU]UZ,system,16
(bootloader) [RUU]WP,system,0
(bootloader) [RUU]WP,system,0
(bootloader) [RUU]WP,system,1
(bootloader) [RUU]WP,system,2
(bootloader) [RUU]WP,system,3
(bootloader) [RUU]WP,system,4
(bootloader) [RUU]WP,system,5
(bootloader) [RUU]WP,system,6
(bootloader) [RUU]WP,system,7
(bootloader) [RUU]WP,system,8
(bootloader) [RUU]WP,system,8
(bootloader) [RUU]WP,system,9
(bootloader) [RUU]WP,system,10
(bootloader) [RUU]WP,system,11
(bootloader) [RUU]WP,system,12
(bootloader) [RUU]WP,system,13
(bootloader) [RUU]WP,system,14
(bootloader) [RUU]WP,system,15
(bootloader) [RUU]WP,system,16
(bootloader) [RUU]UZ,system,16
(bootloader) [RUU]UZ,system,16
(bootloader) [RUU]UZ,system,17
(bootloader) [RUU]UZ,system,18
(bootloader) [RUU]UZ,system,19
(bootloader) [RUU]UZ,system,20
(bootloader) [RUU]UZ,system,21
(bootloader) [RUU]UZ,system,22
(bootloader) [RUU]UZ,system,23
(bootloader) [RUU]UZ,system,24
(bootloader) [RUU]UZ,system,24
(bootloader) [RUU]UZ,system,25
(bootloader) [RUU]UZ,system,26
(bootloader) [RUU]UZ,system,27
(bootloader) [RUU]UZ,system,28
(bootloader) [RUU]UZ,system,29
(bootloader) [RUU]UZ,system,30
(bootloader) [RUU]UZ,system,31
(bootloader) [RUU]UZ,system,32
(bootloader) [RUU]WP,system,16
(bootloader) [RUU]WP,system,16
(bootloader) [RUU]WP,system,17
(bootloader) [RUU]WP,system,18
(bootloader) [RUU]WP,system,19
(bootloader) [RUU]WP,system,20
(bootloader) [RUU]WP,system,21
(bootloader) [RUU]WP,system,22
(bootloader) [RUU]WP,system,23
(bootloader) [RUU]WP,system,24
(bootloader) [RUU]WP,system,24
(bootloader) [RUU]WP,system,25
(bootloader) [RUU]WP,system,26
(bootloader) [RUU]WP,system,27
(bootloader) [RUU]WP,system,28
(bootloader) [RUU]WP,system,29
(bootloader) [RUU]WP,system,30
(bootloader) [RUU]WP,system,31
(bootloader) [RUU]WP,system,32
(bootloader) [RUU]UZ,system,32
(bootloader) [RUU]UZ,system,32
(bootloader) [RUU]UZ,system,33
(bootloader) [RUU]UZ,system,34
(bootloader) [RUU]UZ,system,35
(bootloader) [RUU]UZ,system,37
(bootloader) [RUU]UZ,system,38
(bootloader) [RUU]UZ,system,39
(bootloader) [RUU]UZ,system,40
(bootloader) [RUU]UZ,system,40
(bootloader) [RUU]UZ,system,41
(bootloader) [RUU]UZ,system,42
(bootloader) [RUU]UZ,system,43
(bootloader) [RUU]UZ,system,44
(bootloader) [RUU]UZ,system,45
(bootloader) [RUU]UZ,system,46
(bootloader) [RUU]UZ,system,47
(bootloader) [RUU]UZ,system,48
(bootloader) [RUU]WP,system,32
(bootloader) [RUU]WP,system,32
(bootloader) [RUU]WP,system,33
(bootloader) [RUU]WP,system,34
(bootloader) [RUU]WP,system,35
(bootloader) [RUU]WP,system,36
(bootloader) [RUU]WP,system,37
(bootloader) [RUU]WP,system,38
(bootloader) [RUU]WP,system,39
(bootloader) [RUU]WP,system,40
(bootloader) [RUU]WP,system,40
(bootloader) [RUU]WP,system,41
(bootloader) [RUU]WP,system,42
(bootloader) [RUU]WP,system,43
(bootloader) [RUU]WP,system,44
(bootloader) [RUU]WP,system,45
(bootloader) [RUU]WP,system,46
(bootloader) [RUU]WP,system,47
(bootloader) [RUU]WP,system,48
(bootloader) [RUU]UZ,system,48
(bootloader) [RUU]UZ,system,48
(bootloader) [RUU]UZ,system,49
(bootloader) [RUU]UZ,system,50
(bootloader) [RUU]UZ,system,51
(bootloader) [RUU]UZ,system,52
(bootloader) [RUU]UZ,system,53
(bootloader) [RUU]UZ,system,54
(bootloader) [RUU]UZ,system,55
(bootloader) [RUU]UZ,system,56
(bootloader) [RUU]UZ,system,56
(bootloader) [RUU]UZ,system,57
(bootloader) [RUU]UZ,system,58
(bootloader) [RUU]UZ,system,59
(bootloader) [RUU]UZ,system,60
(bootloader) [RUU]UZ,system,61
(bootloader) [RUU]UZ,system,62
(bootloader) [RUU]UZ,system,63
(bootloader) [RUU]UZ,system,64
(bootloader) [RUU]WP,system,48
(bootloader) [RUU]WP,system,48
(bootloader) [RUU]WP,system,49
(bootloader) [RUU]WP,system,50
(bootloader) [RUU]WP,system,51
(bootloader) [RUU]WP,system,52
(bootloader) [RUU]WP,system,53
(bootloader) [RUU]WP,system,54
(bootloader) [RUU]WP,system,55
(bootloader) [RUU]WP,system,56
(bootloader) [RUU]WP,system,56
(bootloader) [RUU]WP,system,57
(bootloader) [RUU]WP,system,58
(bootloader) [RUU]WP,system,59
(bootloader) [RUU]WP,system,60
(bootloader) [RUU]WP,system,61
(bootloader) [RUU]WP,system,62
(bootloader) [RUU]WP,system,63
(bootloader) [RUU]WP,system,64
(bootloader) [RUU]UZ,system,64
(bootloader) [RUU]UZ,system,64
(bootloader) [RUU]UZ,system,65
(bootloader) [RUU]UZ,system,66
(bootloader) [RUU]UZ,system,67
(bootloader) [RUU]UZ,system,68
(bootloader) [RUU]UZ,system,69
(bootloader) [RUU]UZ,system,70
(bootloader) [RUU]UZ,system,71
(bootloader) [RUU]UZ,system,72
(bootloader) [RUU]UZ,system,73
(bootloader) [RUU]UZ,system,74
(bootloader) [RUU]UZ,system,75
(bootloader) [RUU]UZ,system,77
(bootloader) [RUU]UZ,system,78
(bootloader) [RUU]UZ,system,79
(bootloader) [RUU]UZ,system,80
(bootloader) [RUU]WP,system,64
(bootloader) [RUU]WP,system,64
(bootloader) [RUU]WP,system,65
(bootloader) [RUU]WP,system,66
(bootloader) [RUU]WP,system,67
(bootloader) [RUU]WP,system,68
(bootloader) [RUU]WP,system,69
(bootloader) [RUU]WP,system,70
(bootloader) [RUU]WP,system,71
(bootloader) [RUU]WP,system,72
(bootloader) [RUU]WP,system,72
(bootloader) [RUU]WP,system,73
(bootloader) [RUU]WP,system,74
(bootloader) [RUU]WP,system,75
(bootloader) [RUU]WP,system,76
(bootloader) [RUU]WP,system,77
(bootloader) [RUU]WP,system,78
(bootloader) [RUU]WP,system,79
(bootloader) [RUU]WP,system,80
(bootloader) [RUU]UZ,system,80
(bootloader) [RUU]UZ,system,80
(bootloader) [RUU]UZ,system,81
(bootloader) [RUU]UZ,system,82
(bootloader) [RUU]UZ,system,83
(bootloader) [RUU]UZ,system,84
(bootloader) [RUU]UZ,system,85
(bootloader) [RUU]UZ,system,86
(bootloader) [RUU]UZ,system,87
(bootloader) [RUU]UZ,system,88
(bootloader) [RUU]UZ,system,88
(bootloader) [RUU]UZ,system,89
(bootloader) [RUU]UZ,system,90
(bootloader) [RUU]UZ,system,91
(bootloader) [RUU]UZ,system,92
(bootloader) [RUU]UZ,system,93
(bootloader) [RUU]UZ,system,94
(bootloader) [RUU]UZ,system,95
(bootloader) [RUU]UZ,system,96
(bootloader) [RUU]WP,system,80
(bootloader) [RUU]WP,system,80
(bootloader) [RUU]WP,system,81
(bootloader) [RUU]WP,system,82
(bootloader) [RUU]WP,system,83
(bootloader) [RUU]WP,system,84
(bootloader) [RUU]WP,system,85
(bootloader) [RUU]WP,system,86
(bootloader) [RUU]WP,system,87
(bootloader) [RUU]WP,system,88
(bootloader) [RUU]WP,system,88
(bootloader) [RUU]WP,system,89
(bootloader) [RUU]WP,system,90
(bootloader) [RUU]WP,system,91
(bootloader) [RUU]WP,system,92
(bootloader) [RUU]WP,system,93
(bootloader) [RUU]WP,system,94
(bootloader) [RUU]WP,system,95
(bootloader) [RUU]WP,system,96
(bootloader) [RUU]UZ,system,96
(bootloader) [RUU]UZ,system,96
(bootloader) [RUU]UZ,system,97
(bootloader) [RUU]UZ,system,97
(bootloader) [RUU]UZ,system,98
(bootloader) [RUU]UZ,system,98
(bootloader) [RUU]UZ,system,99
(bootloader) [RUU]UZ,system,99
(bootloader) [RUU]UZ,system,100
(bootloader) [RUU]WP,system,96
(bootloader) [RUU]WP,system,97
(bootloader) [RUU]WP,system,98
(bootloader) [RUU]WP,system,99
(bootloader) [RUU]WP,system,100
(bootloader) ...... Successful
(bootloader) start image[sbl1-3] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-3,0
(bootloader) [RUU]UZ,sbl1-3,100
(bootloader) signature checking...
(bootloader) [RUU]WP,sbl1-3,0
(bootloader) [RUU]WP,sbl1-3,100
(bootloader) ...... Successful
(bootloader) start image[sp1] unzipping & flushing...
(bootloader) [RUU]UZ,sp1,0
(bootloader) [RUU]UZ,sp1,99
(bootloader) [RUU]UZ,sp1,100
(bootloader) [RUU]WP,sp1,0
(bootloader) [RUU]WP,sp1,100
(bootloader) ...... Successful
(bootloader) start image[radio] unzipping & flushing...
(bootloader) [RUU]UZ,radio,0
(bootloader) [RUU]UZ,radio,7
(bootloader) [RUU]UZ,radio,13
(bootloader) [RUU]UZ,radio,20
(bootloader) [RUU]UZ,radio,26
(bootloader) [RUU]UZ,radio,33
(bootloader) [RUU]UZ,radio,40
(bootloader) [RUU]UZ,radio,46
(bootloader) [RUU]UZ,radio,53
(bootloader) [RUU]UZ,radio,60
(bootloader) [RUU]UZ,radio,67
(bootloader) [RUU]UZ,radio,73
(bootloader) [RUU]UZ,radio,79
(bootloader) [RUU]UZ,radio,87
(bootloader) [RUU]UZ,radio,93
(bootloader) [RUU]UZ,radio,99
(bootloader) [RUU]UZ,radio,100
(bootloader) [RUU]WP,radio,0
(bootloader) [RUU]WP,radio,26
(bootloader) [RUU]WP,radio,53
(bootloader) [RUU]WP,radio,79
(bootloader) [RUU]WP,radio,100
(bootloader) ...... Successful
(bootloader) start image[rcdata] unzipping & flushing...
(bootloader) [RUU]UZ,rcdata,0
(bootloader) [RUU]WP,rcdata,0
[B](bootloader) ...... Successful
OKAY [200.187s]
finished. total time: 242.101s[/B]

okay, so the progress bar didn't go all the way, but the command reported "Successful" (check the command prompt output above, I've highlighted it in bold), so let's reboot:

C:\ADB3>[B][COLOR="Blue"]fastboot reboot-bootloader[/COLOR][/B]
rebooting into bootloader...
OKAY [ 0.037s]
finished. total time: 0.037s