Whats most important when learning to skin is to research the actual planes.
What I mean with that is to research how metal looks, how metal looks worn, how paint looks, how paint looks worn (Japanese, German, UK, Russian, US paint wears differently), how each individual plane is worn (where its worn and why), where was the plane used (theatre inpacts on weare), oil leaks (where did the plane have oil leaks?), ect ect.
If you can get a understanding for all of that about the actual plane then you will be ready to start skining. For me as a totally new skinner this was the most time consuming thing. Once you understand how your plane should look then its time to start making theory into practice.
Tex