I am new to modding. I have never made a mod bigger than adding in a house. I would like to edit a texture on the hunting shotgun to make it have a camo decal. I have no idea how to do this and i was wondering if anyone can give or link me a tutorial on editing textures. I have gimp.

This re-texture tutorial is for Oblivion but the principle should be the same for FNV:
http://cs.elderscrolls.com/index.php/GIMP:Retexturing_Oblivion