[OnePlus 5T  Questions & Answers] Sass86oh:
Okay there's a few things remember here after flashing the mokee trebleize pack you should have had the addition of the vendor partition included in your TWRP file system if you have the blue spark TWRP version YOU SHOULD BE ABLE TO SEE THE ADDITION OF THE VENDOR PARTITION WHEN YOU GO TO MOUNT FOLLOWING THAT.

*** Make sure that after the trebleize is installed or whatever don't touch vendor partition ever just leave it alone you don't need to do anything to it it's fine if some ROM needs to update it you don't have to worry about wiping it first because it'll take care of that with the configuration script in the zip file. Even if you want to go back to a regular Android 8. 1 ROM you don't have to worry about the vendor partition the way it's set up the vendor partition that exist inside the one plus 5T that was created for Treble is not seen by any AOSP ROMs so it's going to create its own vendor folder inside of the system partition like it normally does and that you will not conflict because the fs tab does not Mount the new vendor. By leaving vendor alone and never touching it you're just ensuring that when you flashed yes eyes you're not missing something that's essential to the boot up process and it's one thing that you can from any potential uncertainty as to why a treble mage may or may not be booting.

Follow this....

Normal Android ROMs you would usually wipe system data cache D-cache if you were going to install a new ROM that wasn't just an upgraded version of the one that you were already running you'd have to start from scratch always. This is still what you should do it whenever you want to start and flash a fresh regular ROM or treble run always do that but it's how you proceed from here that is a little difference between the two.

With a regular ROM you would just go on to flash the ROM that you want to install followed by Gapps and so on.

When you want to flash a treble image you need to be on a clean fresh ROM that has treble support already good to go in that it can see that vendor partition that was made before and that it contains the binderized HALs needed as well. Because official treble support is still in beta with the OnePlus 5T the only officially supported treble Android 8. 1 ROM is mokee so you always want to start with a clean flash of the Mokee ROM because it also contains the supported kernel and ramdisk and other things as well.

Upon flashing a fresh version of mokee you want to go ahead and Flash no-dm-variety to ensure data is not encrypted because it will lead to a bootloop and the only way to get it to work will be to either format your SD card or to install the no variety which is the way to go just install it every time you flash mokee so that way you know everything is good.

**** I've only had experience using Android 8. 1 GSi so I'm not sure exactly how it would work concerning open G apps because I've only had to do it with two 8. 1 apps I'm not touching the Android P treble ROM until it's official so anything that I'm saying here has nothing to do with that if that's the image you're trying to install that I can't help you with it because I don't honestly know I remember reading something about that changing something different with it and I got to read up more on it.****

So next part depends on whether the GSI you're trying to flash comes with GFS included inside of it some do some don't I think most do but just make sure you read the instructions on the forms from where you download them from where they will tell you about the gf's the ones that I've installed that didn't have gf's on I would install mokee then I would flashed gf's to mokee but I've seen ones where people have flashed the gf's after flashing the treble images well and they didn't have any problem so just make sure you read into that one. obviously if the GS I can change the gf's then don't flash any before flashing or after as it will be flashed when we install it to the image and everything will be.

Which brings us to the last part if you did everything right up to this point then you should have no trouble installing the GSI through either TWRP or fastboot but before you do just make sure you go back to White and you just swipe to factory reset again so that we know that we cleaned out the data and we're on something clean after on encrypting the data if you did Flash no variety but remember don't wipe any partitions you want to make sure that you leave mokee on there. if you do it correctly you won't even need to wipe anything afterwards you should literally just need to be able to install the image into the system partition now and then reboot and it should work fine

It's your choice on how to go about doing it I've had success with TWRP but I've had more success using the fastboot method if for some reason you do try TWRP and it flashes and says success in 10 seconds then just reboot to fastboot mode in just flash it from there because you probably going to run into an issue just factory reset it again before you do it by swiping you don't have to wipe any of the partitions leave those alone.

So either go to the fastboot route which is put the phone in fastboot Point your terminal to the area where your GSI is located on your computer and then type fastboot flash system xxxGSI NAMEXXX.IMG.... OBVIOUSLY REPLACE THE FIRST PART WITH THE ACTUAL NAME OF YOURS. Next press enter and give it a few seconds and then you should see it's a success at which time you can reboot the phone and it should boot after a few short minutes.

If you wanted to use the twrp I believe I saw something about how you have to be on version 3.2.2 or above most likely for blue spark version as I use that one because it has treble support and I can see the vendor image even though I don't touch it. So if you flash it here you want to make sure instead of rebooting to fastboot like before you would just go to install and select image then find the image on your phone select it and then select the system images where you are installing it and then wipe the bar at the bottom for Flash and then upon it finishing you should hit reboot and let the phone boot up the first time set it up and then you can go back and you can go back to TWRP if you want to flash magisk or any other modules or TWRP Zips I've stated first then make sure you move the phone and go back and do all the modifying after otherwise you're getting stuck. I hope this helps feel free to contact me if you have any trouble.