Cut the tongue
Mark the cut post top for a tongue. The tongue should be 33⁄8 inches long and wider at its base than at its shoulders. Make
it 1 inch wide at the base, tapering to ½ inch at the shoulders.
Set a circular saw to make a cut 1½ inches deep, and cut the line at the top of the shoulder. Then set the saw to full depth and cut the other (tapered) line, taking care not to cut too far.
Flip the post over and cut from the other side, then finish the cut using a handsaw. Cut the other side of the tongue in the same way.