in the "files\gamecore\guns\model pack 5" folder, their is another folder called "HANDS_RESOURCE" in here you can find separate folders containing all the different variations of hand textures, to apply one to a weapon, simply COPY one of the "hand_tex.jpeg" files (the variation you want, you can easily determine from the looks of the texture) and PASTE it into the folder of the weapon you want them applied to, it will prompt you to overwrite the existing hand texture, click Yes, remember to keep backups of these texture though, thus the reason i said copy, not cut.
i hope that answers your question.
If however you are talking about creating your own, you will need an image editor, simply draw over the texture as you would like it to appear in game, all hand variation textures need to be named "hand_tex" and be in jpeg format (i used jpeg for easy modding compatibility)
