Add the plywood top to the frame.

The long edge will be flush with the front of the frame, and the short sides will be flush the long plywood sides.

Screw this into place with 1 1/2" screws - put one on each end and two evenly spaced in between. Make sure these screws are going into the 2x4 framing, not the other plywood.

Once you have the plywood screwed in, use 6d or similar nails to tack the edges of the plywood together. Hold them flush as you do this. Pre-drilling holes is helpful for those with less experience.