This thread is for stupid questions!:D
I have made very many of them...
At first you need to download Terrain Editor from HTC Downloads Sites. Skin files are 256 colored bmp files, so you need some art program to make/edit them. For ex. Adope Photo Shop is good!
You must select the skin (what you want to edit) from TE's shape list (Objects): for ex. choice BF109G2 and click "Texture" button. After that all BF109G2 skin files are going to aheditor/terrains/yourmap/savedtxt" folder. Pick them from there!
When you are editing skins, you must at first convert those 256-colored (8 bits) files to 24 bit files. After editing you convert them back to 8 bit files and put them to Terrain Editor's "texsrc" folder.
Use Terrain Editor's Help file to find better instructions!;)