Oh, well, if you want to make a stationary, unplayable piece of scenery, that's not hard at all. You can just replace an existing piece of scenery by going into srb2.srb, looking up which sprite you want to replace, and create a sprite in your WAD that has the same lump name.
To make custom enemies and the like, it would be more complex, as you would need to do some SOCing.