GCam

there is no guide ... 

you need something to decompile the apk - do modifications - compile the apk (i use apk easy tool for this)

after decompiling you get .smali code files you can edit (e.g. notepad++)

for better understanding whats going on you can get (more or less readable) java code with jadx-gui, makes it easy to browse through the methods

for modifications you have two ways:

a) learn smali code and edit dicretly (hard)

b) do something with java - compile it to smali - and try to edit this in (easier but longer way)

what i do is i use android studio and create new classes for modifications (e.g. awb button), then you need to find the right place to call it, hope it compiles, fix errors, hope the app actually opens up, fix errors, hope nothing else got broken, fix again and try again.

there is A LOT trial and error.