Note that each step is successive; screwing up one step means the rest might
be screwed as well.
1. You would need to find a base skin that matches the shape of the skin you want to "make". (No appropiate base skins means you can't continue.
2. Find the offsets of the mips in Texture Finder or from the "Offset Centrals" in this community.
3. Configure the CFG file in skinnerUI by inputting the file name, DXT format, offsets of the mips.
4. Load the base skin with skinnerUI and export a BMP file from the base skin.
5. Modify the BMP with any image-editting programs you have got. Save the BMP with a different name so you don't overwrite it in an accident.
6. Open the BMP you saved in last step with IrfanView, "color swap" from RGB ---> BGR and save with a new name.
7. Load the base skin with skinnerUI and import the modified BMP back into the base skin.
8. Your new custom skin still uses the base skin's file name. So, rename it and place it at a location that children can't reach.
9. To use your new custom skin, make a copy of your custom skin and rename it with the file name of the skin you want to replace. Place your custom skin in the actors folder and overwrite that skin you want to replace.