How to rig an SFM model for Garry's Mod?

I would like some help, please.

A couple of months ago, I requested my own personal Halo 4 Spartan model to use in Source Filmmaker. It got done, and I’ve made some cool stuff with it. Today, I’ve decided to port the model myself into Garry’s Mod to use, but there’s a problem: I don’t know a single thing about rigging/porting and stuff like that.

I’ve used this method to get the model into the game, but there’s a setback: It doesn’t have a skeleton that works with GMod, so I have to use Advanced Bone Tool to pose it. Plus, if I save a session, then reload it, the model vanishes and I would have to pose it again.

**Also, when I display the model in Garry’s Mod, there’s an issue with the textures, or at least on the left shoulder plate.

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I would appreciate any help in walking me through the steps needed. Thanks!