Personal Skin Material and Model file

I’ve seen a lot of personal skins on gmod but i’m trying to figure out how do the gmod model get there own Material and Model file.

Mostly i’m more concern about the model file like for example

eric.dx80
eric.dx90
eric.mdl
eric.phy
eric.sw
eric.vvd
eric.xbox

i was wondering how can i make my own with Male9.