How do I put textures on custom weapon models?

[Aboogie1]

I'm using various weapons from different mods in my own compilation mod so I already have the models and textures. Everything (models, textures and esp) is installed where it should go and I've added all the weapons in the CK, but can't figure out how to attach the textures. I see where you can add textures to the weapon model, but none of the custom textures are in the drop down list. Some of the weapon models already have textures in their list, but when I look at them in-game they have no textures. Is there a simple solution, or a good tutorial somewhere for how to do this?