It is safe to say that most of us will not build a beam like the one in this video. It is also safe to say that most of us will need to make a template sometime soon. So, this video treat is a slick little way to get some template advice: 1/8 inch mahogany door skin.
Why door skin makes great woodworking templates:
It's cheap
It's rigid
You can cut it with a knife
You can fine-tune it with a block plane
The process for cutting the template for this joint:
Rip the template stock to the same width as the beam.
Snap a centerline along the template.
Layout the joint and cut it from the single template to yield two mating template pieces.
Snap a centerline on the beam
Index the line on the beam to the center lines on the template