Step 2: Making the Jig
Start by cutting two straight sections of 2x4s 20" long. Dampen a 2" side on one of the boards and run a bead of Gorilla glue down the center of a 4" side of the other. Clamp and let dry, scraping off any squeeze-out before it hardens.
Now cut a 14 degree wedge from the spine of the jig, 8" from one end. A miter saw makes this easy, but you can cut it by hand, too. For a triangle with two sides spanning two 2x4s (actual depth being 5") the short side will equal 1 1/4". Draw this triangle on both sides of the spine and cut carefully, watching both sides as you go.
Tape the acute angles tight against each other. Urethane glues excel at binding wood by the end grain-- dampen one side, apply glue to the other, then stand the jig upright, letting the tape act as a hinge and the weight of the 2x hold it in place. Let dry, scraping off any squeeze-out before it hardens.
Mark a line about 1/2" from either side of the joint. With a rasp, round the edge back to the line to make a smooth transition.