This guide is split into 3 parts, each part assumes knowledge from the previous section
This is specifically a guide for making a Souper Secret Settings compatible resourcepack, however it also covers a lot of general post shader topics, so you could also use this as a guide for making vanilla post shaders (e.g, to then apply through the transparency.json from fabulous graphics)
There is an existing Google Doc, which covers many of the same topics, although it is of course limited to just what you can do in vanilla - Souper Secret Settings allows for some more features and QoL here and there
Souper Secret Settings also supports resource packs made for Perspective. For specific information related to Perspective, you can visit the Perspective Wiki: Super Secret Settings.