He first created the jig using some thin-ripped plywood. You’re creating a wooden ‘saddle’ that can be fitted over the workpiece and clamped down. You want to make sure that the two top pieces of the jig are square. The width of the inside dimensions of the jig needs to consider both extremes of the saw’s base plate travel as it cuts the outer edges of the dado.
Once the jig is clamped securely in place, all you have to do is start the saw at one side of the jig, and cut your way across using a very close sequence of cuts. This jig will allow you to quickly cut a number of notches in a 2×4 to the same width and depth. And, knocking out the thin cuts from the joint is at least as satisfying as popping bubble wrap.