I know by now this is a really old thread, I came upon it while I was trying to remember how I laid out and cut the step laps for my barn. The layout didn't seem especially difficult but like everyone else I never found a way to make the cuts with power tools. I had 68 joints to cut and made it a sport to cut them quickly and accurately. The best way I found to cut them was to start as suggested earlier, but with a variation for the pocket.

Saw the shoulder of the seat with a good hand saw and chisel out in between.

Now, don't go for your framing chisel. Get the corner chisel. Its amazing how good a corner chisel is for hogging out the bulk of material. Forget about the corners, it'll work fine but you don't really save that much time with a corner chisel there. Use whichever chisel you have in your hand for the corners. It's also good for any mortise that's not drilled first. Get a sturdy one and hit it hard and fast and don't worry about being nice to your chisel. Stay away from the lines as you can clean up better with a flat chisel.

Drink coffee, maybe more coffee.
You've got a long row of joints that are just alike so you don't have to rethink each one and you can make good time here.

Hope this is helpful and thanks to all for the tips!

Mark