Manually installing the OS requires you to unlock your phone(bootloader) to allow installation of the fpos or any other os. In my case, this process made is so many apps would not work. To get the apps working again I had to lock the phone(bootloader). Locking the phone/bootloader comes with a risk of breaking your phone beyond repair.
If you still want to attempt it there is this guide:
But if you’re gonna try it, I would suggest looking at my other post: Can't lock bootloader
I would also advice you to read other posts about unlocking/locking the phone/bootloader etc. This might help you avoid issues.