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