most of it is all trial and error
but the big thing is the alpha layers if you want a slight shine to your repaint make the alpha a light gray how ever if you don't want some parts to shine in the alpha paint that part white
if you are making a repaint with bare metal areas make sure you do the same to the alpha but color the less reflective parts in a light gray and the bare metal area in a darker gray eg 55% gray (R137 ,G 137 ,B137)

but one thing for sure you will have fun painting i started 3 years ago as i wanted a repaint done for me but no one would do it and im still hooked now :D