Comment by andrewflnr

The biggest problem I see is that the part of the board you need to cut off intersects the ground. If you're leaning the board on the ground, you need to somehow trace a line parallel to the ground several inches above it. Hopefully you can solve that by using a spare chunk of 2x6 and tracing along that. Similar story for the vertical joint.

I don't know if that's what the author meant, though.