How to repack Magisk boot image? (Magisk installed)

everyone. I have a OnePlus 5t, updated to Android 9 and Project Treble. I have my own Magisk module for my own tweaks, but I am not satisfied. I found that I need a patched init.rc in order to have one script of mine run when I need it. There is an official howto chapter dedicated to an "overlay.d" directory that can modify files on the root of the filesystem, say /init.rc, https://topjohnwu.github.io/Magisk/guides.html#root-directory-overlay-system , however, this "howto" is terse and hard to understand for people not familiar with Magisk's internals. There is also a script that should help to repack the boot image https://github.com/topjohnwu/Magisk/blob/master/scripts/boot_patch.sh , but it is relying on certain commands not in that directory. By trial and error I have found a way to extract "magiskboot" from the magisk's install Magisk-*.zip, and I also found that this magiskboot can unpack the /dev/disk/by-name/boot partition. (I know, that is probably insane, a partition is not a file, but somehow magiskboot manages to navigate it by itself.) But that requires running that command on the phone itself (with magisk's su), and I am not sure how valid this is. Trying to run the "boot_patch.sh" by first copying it to the phone, and using the extracted rootfs.cpio seems way beyond my ability, and I am also very unsure whether this can be done with the system booted, or I should do it from TWRP, or whatever. So, a decent guide (for dummies) on how to patch the boot image would be appreciated. I only need to add two files: custom.rc, with the service description, and a custom.sh with the service code itself. Help would be appreciated!