Dirty flashing from OOS12 based version is fine. The required firmware is included in the ROM.
When coming from OOS to Nameless-AOSP OOS13 based ROM you need to have OOS13 installed.
For OOS12 based build the required firmware is C.62-C.65 IN BOTH SLOTS! DO NOT SKIP THIS!
Working brains, common sense, lots of coffee
Developer options enabled: Settings -> About phone -> Tap Build number at the bottom seven (7) times
ADB enabled: Settings -> System -> Developer options -> USB debugging
Required images: dtbo.img, vendor_boot.img, boot.img (put these in the same folder with platform-tools)
Get images from downloads page
ROM zip (put it in the same folder with platform-tools)
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.
Reboot your phone to fastboot mode
Open cmd and go to the folder where platform tools are (DO NOT USE POWERSHELL!)
Enter following commands by copypasting one by one:
fastboot flash dtbo dtbo.img
fastboot flash vendor_boot vendor_boot.img
Temporarily flash recovery:
fastboot flash boot boot.img
Reboot your phone to recovery mode, click "Install update" -> "ADB Sideload"
In cmd enter command:
adb sideload Nameless-AOSP_xxx.zip
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"info
Progress on your pc will stop at around 47%. It is normal behaviour.
Click back arrow in recovery, then click "Reboot to system"