Drill Holes in the Wood
Using a drill with a 1-1/2" spade or forstner bit, drill two holes (offset them) down one inch on each side of the center point, between the two lines. Do this on one side of each piece. Finish drilling through each hole you made with a 1/2" drill bit. Take the two vertical legs (21") and measure over 1-3/4" and 3-1/2" from each end. Carry these marks across with a speed square. Do this on the opposing side of both legs.
Follow the directions above for each end of one side of each leg.