Cut and sand all lumber. If you don't have a power saw, have the lumber yard cut your lumber before you load up.
Set your four beams in a rectangle with the 33-inch beams inside the 48-inch beams. The beams should lie on their narrow (2-inch wide) sides.
Connect the beams with two screws at each joint. In each case, screw through the face of the longer beam and into the end of the shorter beam.
Set the plywood sheet on top of the frame with all edges flush. Screw it in place with one screw per corner. Reinforce with an additional screw in the middle of each side. For each screw, drive through the face of the plywood and into the edge of the beam beneath.
Set one post in the corner of the rectangular frame, in the angle formed by two beams.
Attach the post with three screws, drilled through the plywood and into the end of the post. Reinforce with one screw drilled through each beam and into the side of the post.
Repeat step six for the other three posts.