You'll have better luck modding Steam than on Xbox. There are tools that exist that make it possible but they're hard to find, and the main modding tools do not support the Xbox version out of the box.
As for modding on PC, there are plenty of guides out there for all kinds of stuff. If there's something specific you want to do, throw that out there and we can get started. I'm not terribly active but I do come by, and I'll happily help because it's not like there's a whole lot of us.