Lay the large plywood sheet out flat on the ground. Measure out and mark, from top to bottom, the strips that will form the irregular panels. There will be inch-wide strips laid flat, parallel to the plywood, with 3-inch-wide strips standing on their edges, separating each one.
Cut some sections of dowel that will act as spacers, raising each of the flat strips up off the backing plywood to create grooves of different depths between the side walls. Cut pairs of dowel spacers at 3 inches, 2-1/2 inches, 2 inches and 1-1/2 inches. Number them 1, 2, 3 and 4. The depths of the wells in a quadratic diffuser are calculated to minimize resonance.
Place a pair of identical spacers in each flat strip section marked on the plywood. From left to right the sequence should follow "1, 2, 4, 3, 3, 4, 2" and so on. Repeat as many times as necessary to fill your board.
Glue the spacers down onto the plywood board once you have the sequence correct. Screw down the inch-wide strips through the spacers into the plywood backing. Take the 3-inch strips and pipe a strip of wood glue down the thinnest edge; then stand them up on that edge between the flat strips.
Allow the glue to dry, and your diffuser is ready to be hung on a wall in your studio.