Need help with a gun mod (UNSOLVED)

Hello, i am working on a gun mod called Simple yet Powerful, and im stuck on making skins and attachments for the guns, i really want it to be in the first version but i dont know how to change an item's texture/model dynamically, if anyone could help, i'd really appreciate it, also please explain it in a simple way, i dont know java.

PS: Sorry for bad english, im from Poland.