I wish I could recommend something but I learned it years ago and I honestly don’t remember where from. I’ll try to put together a modern version of my old LUG talk on the subject to show people how to get started.
I know there is an install, postinstall, and remove scripts but don't have a clear idea of when exactly they will get called and what "interface" they must implement.
It would be really helpful to have a "package lifecycle" state diagram like this https://vuejs.org/v2/guide/instance.html#Lifecycle-Diagram that shows all the states and events that "fire" based on .deb scripts and metadata.
I skimmed the New Maintainers Guide for a rough idea of what to do and then used the Debian Policy Manual with the various debhelper manpages for specific references of what each field/script meant. I also dig into the source of some packages to see how they were packaged (for example, when I was creating an archive keyring). The two areas I had the most trouble with were deb triggers and debconf, which only seem to be documented on external sites.
New Maintainers’ Guide: https://www.debian.org/doc/manuals/maint-guide/