With all of the parts cut and the french cleat ripped in two and ready to go, the next step is to mark the locations for drilling the holes for the two iron bars. The holes locations need to be precise, and the holes should be drilled to a specific depth with a forstner bit in the next step.
On the inside, front faces of one of the two vertical 2x6 boards that comprise the side frames of the unit, make a pencil mark three inches down from the top and 1-1/2 inches in from the front edge. Then, make a second pencil mark 1-1/2 inches up from the bottom and 1-1/2 inches in from the front edge.
Repeat with the inside face of the opposite board, in the same locations.