Taking your time is absolutely essential. Look for the stuff that has reskin kits (Jointed Rail has a lot)- these are almost like fill-in the blanks reskinning. There is a layer that you paint, and there is a "bake" layer that overlays texturing and details. Frequently, there are other detail layers, grime layers, etc., that take care of the hard stuff. Maybe try something like this first. I use Photoshop CS5, so I don't know if the layering will work in free stuff like GIMP. Other reskin kits (McHawkman for example) have a zillion layers for the individual details, and are much more time consuming. Finally, to take a raw TGA and create a new skin can be difficult, to say the least.
Practice, practice, practice. Expect to churn out some "not so good" ones at first. Don't expect to turn out reskins in 5 minutes. It won't happen. It is a time-consuming process. You'll get better at it the more you practice.