It depends on your version like GOG version should also work. But I have no idea what he means. @jamesPLANESii

You don't need steam do you? You just have to look in the game files. @KDS

I'm assuming you mean how to XML mod.
Step One: Buy the game off Steam.
Step Two: Download a XML modding program, the Overload mod is best.
Step Three: Experiment on using it.
If you mean downloading a mod. Click this link.
If you meant how to make a mod Then Click this link.

What do you mean by mod
Xml mod or make a mod