How to hex models?

Wasn’t sure if this went here or in the models/skins section, but oh well.
I recently released a reskin of Vivallion’s personal model, and I need to hex it so that people don’t overwrite his with mine. I’ve tried renaming the model files, but when I spawned it in-game, it was invisible.

It does go in models and skins.

We really need a sticky on this subject to keep this question from being asked so often.

Good tutorials are:
http://www.facepunch.com/showthread.php?t=664977
http://www.facepunch.com/showthread.php?t=395931
http://www.facepunch.com/showthread.php?t=153186

Sticky please !