That gives us something like this:



So I snapped lines instead of using a combo square, because the piece had a little bow in it, and sometimes it's just easier.

Saw the diagonal rafter tail portion (or axe it as Will mentioned).



Clean it up with a chisel or something. Ideally, try to not dish the surface out, because the next part is referenced off that surface.



The next part is the way that I was taught, and it makes sense to me. I've tried to think of an easier way to do this, but haven't yet. So, holding chisel vertical, put corner of chisel somewhere around middle of step part, line up the top of chisel parallel to the surface below (because they're in the same plane). Keep the chisel maybe 1/4" in from the line, because it's going to want to wander.



Do the same on the other side (you're severing the end grain), then chisel out waste in middle. Repeat repeat.



Be careful as you don't want to undercut the triangle that's left in the middle. As you start to get closer, you can set a combo square a little less than what you want (less than 2"), to check for perpedicular off the rafter tail cut. Pare and chop till you get it right on.

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