For OnePlus 8/8 Pro & 8T/9R

Prerequisites

Danger
Required firmware for OOS13 based builds:

8T: F13 or newer
9R: F19 or newer

DO NOT SKIP THIS!

If you are going to dirty flash from OOS12 based build, you need to update your firmware first. Wishmasterflo has made a firmware flasher that checks the RAM type for you and flashes the firmware to both slots. You can download the firmwares from here:

8T firmwares
9R firmwares
Danger
For OOS12 based build the required firmware is C.33-C.36 IN BOTH SLOTS! DO NOT SKIP THIS!
Note
8 / 8 Pro users can dirty flash from OOS12 to OOS13. The required firmware is included in the ROM.

Working brains, common sense, lots of coffee

  1. Latest platform-tools

  2. Google USB driver

  3. Developer options enabled: Settings -> About phone -> Tap Build number at the bottom seven (7) times

  4. ADB enabled: Settings -> System -> Developer options -> USB debugging

  5. Required images: boot.img, vbmeta.img, vbmeta_system.img and recovery.img (put these in the same folder with platform-tools)

  6. Get images from sourceforge

  7. ROM zip (put it in the same folder with platform-tools)

  8. Unlocked bootloader

Note
If you are currently on OOS11 it's better to unlock bootloader before updating to OOS12. For unknown reason some devices cannot be unlocked in OOS12.

  1. Reboot your phone to fastboot mode

  2. Open cmd and go to the folder where platform tools are (DO NOT USE POWERSHELL!)

  3. Enter following commands by copypasting one by one:

    fastboot flash boot boot.img
    fastboot flash --disable-verity --disable-verification vbmeta vbmeta.img
    fastboot flash --disable-verity --disable-verification vbmeta_system vbmeta_system.img
    fastboot flash recovery recovery.img

  4. Reboot your phone to recovery mode, click "Install update" -> "ADB Sideload"

  5. In cmd enter command:

    adb sideload Nameless-AOSP_xxx.zip

  6. Wait until your device shows Step 2/2 and back arrow appears. Click back arrow, then click "Factory reset" -> "Format data/factory reset" -> "Format data"

  7. Click back arrow in recovery, then click "Reboot to system"

Note
While sideloading, progress on your pc will stop at around 47%. It is normal behaviour.