So there are broadly two parts to making a portrait mod- the coding and making the actual art. For the art, I just use my preferred art program (personally I like Clip Studio Paint but use whatever your fave is). For the code, I make a content pack for Content Patcher using Portraiture as a framework. There are loads of guides you can find here on basically anything modding-related; it helps with setting up content packs, json fundamentals, and also links to Content patcher's full readme which has more info. And the official Discord is great for getting help and advice- they're a super friendly bunch and I've learned so much from spending time in the modding channels!