https://litherum.blogspot.com/2019/03/addition-font.html

You’ll want to read this if you want to understand OpenType well enough to make a proper Senior Quikscript font. Most OpenType substitution doesn’t get any fancier than “replace f f i with ffi_ligature”.

There’s also the following, which I probably linked to here before:

https://ansuz.sooke.bc.ca/entry/131