Usually you would join a group and bring your skills with you and work on said project etc.
I don't really see the word hack used very much anymore, we use mod almost exclusively now as it refers to changes made, modifications. Hacking is more used for gaining unauthorised access. However a altered game is also called a hack when uploaded on here. I am now not so sure what the preferred nomenclature is.
It's not so much resources to learn, but instead you bring your knowledge of either programming or some sort of 3d graphic design to work on textures etc.
Some are self thought and doing it as hobby, others self thought and are doing it as a profession, much like anyone that has a computer science degree and also working in industry, could in their down time help out with textures for a mod for an emulator for example. Many come from different backgrounds but have similar skill sets depending on what they work on, a pixel artist is like gold dust these days compared to a 3d artist for example