I bought this sander a few years ago from Harbor Freight. There was a dizzying array of models to choose from. This one had an adjustable handle. When laid flat, the sander can be laid on its back, and it's quite stable. I figured I would convert it into a bench sander, and this feature would save me some work. Even if your belt sander does not stand on its own, this instructable may still help you to attach the all-important fence. See, the fence must be square to the belt if you want to create and/or retain nice, square edges.
Step 2: First attempt
This was my first attempt at attaching a fence. It worked good enough. I used it for three years, and it got the job done. But there were a couple of annoyances with it. As I used it, it "broke in" and the fence tilted. Once it finally settled in, I resanded the face of the fence straight again. That's the most important bit. But the top surface of the fence was no longer in a parallel plane with the belt. This plays tricks on your eyes when you're trying to sand stuff. So I decided to do it again, and to do it better.
Step 3: Basic Problem
The essence of this project is to get the fence to be secure and square. So here's what we're looking at. The belt rides over a flat metal plate. I want the fence to be as close the the back edge of that plate as possible, and still be secure. There's not much there to support the fence. At first glance, this looks like mission impossible. But with the help of physics and some shimwork, it is not as daunting as it may at first seem. All we need to do is support the fence at the four corners, leveling it with shims.