To make successful miters you have to get two things right - you have to have a joint that adds up to 90° (the two sides don't necessarily have to both be 45°, as long as they're close and add up to 90°), and you have to make sure the opposite sides are exactly the same lengths using stops. This is a design that probably accomplishes these goals pretty simply:
He put a corner from a piece of plywood on what looks like fairly standard sled. He used this "factory" corner to guarantee a 90° corner. You can see how he did the stop.