People have posted various templates on this BB; you can do a search.
I started my 4th FG skin (The Deacon, which has already been submitted) using a P-51B panel lines template posted by Skychimp. He may have traced it from the default ("Old Crow") , as the lines matched exactly; he could advise on this. Anyway, without his help, I never would have gotten started - thanks Skychimp. :-)
Once I got going I digitized plans from a book I have, and sized them for the skin. I used these to completely redo the panel lines. Another advantage of having my own digitized plan was that I could trace the rivet lines from it, for the rivet layers. Adjustments were needed to match the 3-D model, especially for the sides, but these plans saved a lot of trial-and-error.
Note that a common skin paradigm seems to be a layer of black associated with another layer of white, offset by 1 pixel diagonally. Opacity can then be separately adjusted for each. As an example, my "rivets black" layer is currently 6% and my "rivets white" at 2% for my Deacon skin. The experts then have a whole series of elaborations on this such as additional gaussian blurred versions of these layers, etc. Also, you can improve on existing practice if you concentrate on photos of real planes, instead of copying what others did. For example, although my first skin has imperfections, I feel I did a relatively good job on the underside weathering, because I did it from a photo, and other better skinners apparently didn't , even though their skins overall may be preferred.
