There is no set way of modifying kits, no rule book or well anything. It is generally down to the individual. Have an idea of what you want to do, get a sharp knife and do it. The best advice I can offer is Google images. Before I build anything, I spend quite a bit of time looking at how other people have done it. From that I either try an replicate or I go my own way. Generally I go my own way using ideas I have got from other model makers. The one thing that is standard across the board when doing this is.............

and lots of it.
Even a standard kit requires it.
My Sea Venom used about two tubes just to get it looking presentable.
Matt
PS, forgot to ask, what are the kits you have??