Clean off both the particleboard and plywood with several drops of liquid dish soap and a soft, damp sponge to remove any dirt, dust or contaminants that could interfere with the glue's bonding process. The dish soap is a degreaser that removes dirt without harming the wood. Lightly rinse the soap off and then dry both pieces with a rag. The pieces should be completely dry before continuing.
Apply epoxy glue to the plywood so it is entirely covered for best results. Press the plywood tightly against the particleboard and wipe away any excess epoxy with a clean rag while it is still wet.
Clamp the plywood and particleboard together so they're held together as the epoxy cures. It usually dries in a few minutes, but leave the clamp on for at least an hour to make sure the pieces are secure before moving them around.