Cut off a 21-in.-long board for the shelves, rip it in the middle to make two shelves, and cut 45-degree bevels on the two long front edges with a router or table saw. Bevel the ends of the other board, cut dadoes, which are grooves cut into the wood with a router or a table saw with a dado blade, cross- wise (cut a dado on scrap and test-fit the shelves first!) and cut it into four narrower boards, two at 1-3/8 in. wide and two at 4 in.
There are a few things particleboard is NOT. It’s not medium density fiberboard (MDF)—a material with greater density and weight composed of more uniform particles. It’s NOT oriented strand board (OSB), a material composed of large wood chips and strands that’s structurally equivalent to plywood. It does NOT have great nail or screw holding ability, nor is it all that water resistant; water can quickly cause the material to swell and lose structural integrity. But if you need something flat and cheap for use in a dry place, particleboard will do you proud. Learn how to make a plastic laminate tabletop with a particleboard substrate.