Also, to make a skin will depend on the software the skin is applied to.
Usually, a skin is basically broken up into peices. One peice will be the upper left corner. To make that peice, you create an image to go in that corner. Then you create the upper right corner's image. Then the middle, etc. The software links it all.
Some applications that you can skin are very complex, and require modifying code to go tith things like clicking effects and slider bars. Others are much simpler and only require a basic shape with no individual peices. I think quicknotes was a basic shape design. I don't quite recall, as I haven't skinned it in years.
Download the application you want to skin and then search for the skinning guide to it. Usually it is an attached file. Sometimes it is on the application's homesite. Sometimes you just have to ask, since it may be hard to find and not intuitive