Second step
1. specify the base rom for porting and fixing later ... copy the entire contents of the system, vendor and boot.img it to the sdcard
2. so that porting runs smoothly, try to find the same android version along with the kernel version, and of course similar or equivalent chipsets too ... e.g. your base rom is pie with kernel version 4.4.xx, sdm636 chipset then porting the ROM is the equivalent of yours or the closest version and the chipset
3. exception: Do not try to porting ROMs from Samsung, Xperia, stock ROMs ... because they have their own modules that aim to hardward belong to each vendor ... then it's a bit difficult for fixing it later
OK .. if all the conditions have been prepared ... then:
-FULL BACKUP FIRST ROM YOU WERE USED WITH TWRP
- unzip rom that will be ported
- make sure there is no firmware update (tz.mbn, aboot.mbn, etc.) if there is a delete along with the caller script in the updater script.
- Delete all vendor file
- unpack your first boot.img base rom, and boot the img rom you want ported ... (how to unpack repack boot.img in my post before).
- Copy vendor.new.dat.br, vendor.patch.dat, vendor.transfer.list from base rom to folder port.
- now to the meta-inf rom port. Edit updater.script (if the top line says get.prop blablabla assert .. then delete assert until the line is complete) (make sure the partition format is correct ... if you encounter a system, data, and cache ext4, then make sure the rom port updater script also says ext4 ... if it hasn't been the same, then change it to the same)
- zip the ROM port that you have modified boot.img to (can be with app. zipper if using a cellphone, or 7zip if using a computer) try flash with twrp (make sure it has a wipe system, data, cache, and dalvik) - wait until boot (about 15-20 minutes)
#note
if the boot fails due to different base and port chipsets, maybe you should look a little at: cmdline (spilled), ueventd (ramdisk)