[Google Pixel 2 XL ROMs, Kernels, Recoveries, & Oth] MrBrady:
My path was a little different (and maybe overkill) compared to the one above. I was on AOSiP, so I wanted to make sure I started with a clean base. Everything is working for me, with one minor exception - Google Markup (to edit screenshots). I think it's an issue with the Gapps package I'm using.

Download your ROM, Gapps, TWRP img. Optional downloads: TWRP zip and Magisk. I used a combination of BiTGapps, BiTGapps add-ons and Razorlove's Google Fi zip. You can also use OpenGapps (see the OP for details).

STEP 1: Flash the August Factory Images to both slots. This assumes you've downloaded the factory images from Google and have already setup ADB and Fastboot. I'm on a Mac, but I don't think the commands are too different if you're on a different platform. Windows probably uses the .bat file instead of the .sh file.

1) On phone, boot into TWRP and do a factory reset

2) On phone, boot into Bootloader and plug into your computer. (I've noticed on my Mac some apps can mess-up this process. Like Adobe XD for example b/c it's looking for USB devices to preview designs on.)

3) On the computer, go to terminal/command line and type 'fastboot --set-active=b', then 'reboot-bootloader'. Your phone will reboot to Slot B

4) THIS WILL WIPE YOUR PHONE'S STORAGE > On computer, still in terminal, navigate to your factory image folder and type './flash-all.sh'. When finished your phone will automatically boot and should be sitting on a setup screen.

5) Reboot phone and hold down volume - button to enter bootloader

6) On the computer, go to terminal/command line and type 'fastboot --set-active=a', then 'reboot-bootloader'. Your phone will reboot to Slot A

7) THIS WILL WIPE YOUR PHONE'S STORAGE > On computer, still in terminal, navigate to your factory image folder and type './flash-all.sh'. When finished your phone will automatically boot and should be sitting on a setup screen.

STEP 2: Flash your ROM and Gapps

1) Reboot phone and hold down volume - button to enter bootloader.

2) On the computer, go to terminal and navigate to your TWRP .img file, then type "fastboot boot [insert twrp.img file name here]". You phone will boot into TWRP. Let TWRP startup and then disable screen time-out in the settings. You're on Slot A now. (you can tell by going to the reboot menu in TWRP)

3) On the computer, go to terminal and to your where you downloaded your ROM files. Then type "adb push [insert file name here] sdcard". This will transfer the designated file to your phone.

4) On phone (still in TWRP), do a factory reset (don't need to wipe internal storage). Then flash the LineageOS zip. Once finished reboot to bootloader.

5) On the computer, go to terminal and navigate to your TWRP .img file, then type "fastboot boot [insert twrp.img file name here]". You phone will boot into TWRP, but you're on Slot B now.

6) On the phone (still in TWRP), flash your Gapps zip(s). If you want to flash the TWRP zip, do it now. I choose not to b/c I don't think you can take Lineage OTA updates if you have TWRP installed.

7) Once zips are done installing, wipe dalvik/cache and reboot.

8) Device should boot into Lineage OS.

9) Setup your phone

10) If you want Magisk, fastboot boot TWRP from bootloader and flash Magisk.

Hope that helps. Enjoy!