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Step 1: Arms
One arm has two 3/8 dia holes drilled in it 6 1/4 inches from the end and 3/4 inches from the top and bottom.
Use these holes as a guide to drill the upright and other arm after everything is assembled.
Cut two pieces of 2x4 3 1/2 inches long for the spacer blocks.
Screw the arms to the spacer blocks so there's a gap in the arm assembly for the upright to fit through snugly.