[P8 Lite (2017) Discussion] XDA XILFY:
Well, I finally got it all working.

You know the risks, don't blame me if something goes wrong, I did these exact steps to manage to root my phone without breaking it. C432, Dual SIM and PRA-LX1.

Have everything stock before following this short guide: Firmware, all stock recovery etc.

Installing the ROM and GApps

First, download necessary GSI files.

(Personally I got Official Oreo Resurrection Remix, here).

(Although you could find yourself using virtually any other GSI, I like the look and feel of RR, though I'll be changing my boot animation with root). Many people would probably recommend you to use something like an AOSP, or LOS which I was also considering on installing. They may also come bundled with GApps, which would negate the later need for installation or download of separate GApps.

This may include GAPPS but you should understand this from the GSI page, so for example on RR I had to download both the latest base ROM .img and the micro GApps.

So to do this I had to use the right architecture (For the P8 Lite 2017, this is ARMx64, do your good research, since my problem was getting this wrong with my PRA-LX1 (Mine was A-ONLY), this stands for the base ROM and GApps.

After this I actually didn't use TWRP as user [MENTION=8461420]ppdr07[/MENTION] advised. I followed the guide he linked us to and did as so, without following the -u command, since it wasn't working for me (didn't affect anything, try it, if it doesn't work, just take that bit out).

The ROM ended up installing and posting through, everything set up perfectly.

After this I was like "Huh, so what now? How am I to flash GApps on this without TWRP?". I went for it and booted into fastboot where I flashed Askuccio's TWRP for hi6250 devices. At this point I worried that the ROM wouldn't post again - though it did, so from there I enabled dev options and switched on advanced power options just for ease of usage. From there I rebooted straight to recovery which posted fine, too.

At this point, you need to flash the GApps. Go for the one that the original GSI page recommends. Flash it to the System Image partition. Don't wipe anything. At this point I recommend that if you realise that you forgot to transfer any files to the SD card, you should do it from the system. It works better compared to the MTP on TWRP. Either way, that should work properly. Now, reboot the system, and it should work perfectly.

Enjoy, it took me months to get to this point lol :D

Rooting Your New ROM With Magisk

(Before installing this consider the other root options. I believe there's SuperSU and another possibly that I'm less aware of. Either way, in my opinion Magisk comes with the most benefits so I'll be helping you out with that).

After this I ended up flashing Magisk in order to use Snapchat and other apps similar to it. Firstly you'll want to set up Google stuff which has been newly added to the system, and then you'll want to go to chrome and download the Magisk Manager .apk from here. The main and most reliable place to get it is from the creator's project page found here, all main links stand out in the middle fairly well.

(Remember to enable downloading of unknown apps for Chrome in it's app settings, otherwise it won't download or install.)

If it still doesn't work at this point, I ended up holding down on the Magisk Manager link and pressing Download Link (or something to that effect). From this point just install it as you would any other out-of-store app.

Now, you'll need to download these two files from the Magisk page onto your computer: Stable Installer and Uninstaller. Next, while booted into Android, copy them over to either Internal or External storage and when it's done, reboot into TWRP again, either by advanced power menu or using the button commands for recovery. Personally, I think I need the power options with this phone.

In TWRP, simply go to Install, navigate to /externalsd/***[WHEREVER YOU PLACED YOUR MAGISK FILES]***, then install the Magisk installer .zip. If unchanged, the filename should be something like "Magisk-v17.1.zip". Install it, then give rebooting a go. Hopefully it'll post, if not, go back into TWRP and run the Uninstaller .zip, it's saved me a bunch of times.

Unblocking Apps That Say Something Like "Your Phone is Rooted or Modified, no access blah blah blah..."

(Snapchat is the example used here. It won't work upon login if you don't follow this step. Some apps don't work with this alone however, though somebody may have produced a separate guide or easily installable Magisk module with a solution for it. One that won't be fixed singularly with Magisk Hide is Google Pay).

After posting, if you need to unblock Snapchat or something, go to Magisk Manager (which should now be more green), open the top left context menu, tap on Magisk Hide, and search "snap" until you see the Snapchat logo pop up, then select it, then back out with the back button. It should stay selected in Magisk, and the app should now be usable and shouldn't give you any annoying errors, if not give it a reboot and see if it's solved.

Good luck!