You learn it by doing it.
Don't expect the manual to explain you how you can do your own photoshop app, however, I learnt B+ by just doing an app with a real purpose.. that sortof forced me to take it seriously.
The only thing I advice is to develope your own style. And try to make a good looking/working frame&mainloop system. The mainloops in the examples are a bit messy.
My usual frame/mainloop: (note that the eventcheck can be put in a seperate function (since the app is a global), and you could also use a 'Select' here)
Global app=CreateWindow("title",0,0,1024,768,0)
Global quit=False
Repeat
WaitEvent()
If EventID()=$803
If EventSource()=app quit=True
EndIf
; ------- your stuff below --------------
; ---------- until here -----------------
Until quit
FreeGadget app
End