This isn't easy.....
You need software to extract the 3d model from the other game files first.
I am using Zmodeler myself to extract meshes from GTA but they often get messed up.
When i finally have a 3dmodel extracted in milkshape then the regrouping and remapping (uvmap) starts.
Really alot of work....
I suggest you start with a
tutorial first and use a 3dmodel from the internet.