Hi ace888. You've got nothing to lose by getting GIMP. I find it so easy to use compared to Photoshop Elements and I use it to make all of my decals and to do the little bit of skinning/adapting that I do.
As STORM has suggested you'll need to open up each individual BMP from whatever skin you are trying to adapt and also open up the stock BMP to compare and contrast. Even better, you can copy each different BMP (i.e. third-party and stock) into a new file (save as a PSD) to make your first layered templates and then either turn on and turn off a layer to see the layer below or set one layer at, say, 25% opacity to do the compare and contrast thing. That's how I adapted the F-4M RAF Camo skin to make it fit the F-4E. You'll pick it up easy enough. Oh, and the SimmersPaintShop (as recommended by hgbn) is worth a visit as that's where I learned how to make decals - I was totally clueless at first!