Construct the frame. Use the 4 foot long 2-by-4 boards to make cross beams by nailing them between the two 8 foot 2-by-4 boards that form the end beams, at 1-foot intervals. Hammer two nails into each end of the cross beams.
Attach the top of the stage. Lay the plywood on top of the frame and nail it down.
Add the posts. Flip the stage over so the plywood is touching the ground. Place a 4-by-4 piece of lumber on the inside of each of the four corners of the stage as well as one in the inside center of each end beam. Use the drill to drill two 1/2-inch holes through each 4-by-4 and through the end beams.
Secure the posts. Pass bolts through the holes you drilled. Place a washer on the end of each bolt and follow it with a nut. Use a socket wrench to tighten the nuts securely.
Paint the stage. Flip the stage over so it is sitting on the posts. Paint the stage with black outdoor paint.