Make evenly spaced marks along the two 2"x4" wood strips at the points where the shelves will be placed.
Set the height of the table-saw blade to 1 1/2". Tilt the blade of the saw to 45 degrees. Cut through each pencil mark in the two strips of wood.
To make wider slots, use a chisel and a hammer to cut out the area between each set of slots.
Screw the two strips of wood to either side of the 3/4"-thick plywood board.