Cut the 4-by-4 posts so that they are 5 feet long. Measure the length, mark it with a pencil and cut with a circular saw.
Mark and cut the 6 foot long 2-by-4 board into four 1 1/2-foot lengths.
Lay one 4-by-4 post down on the ground, and place one of the 2-by-4 boards across it on its 4-inch side. Set it so that the board is centered on one end of the post forming a T. Screw the board to the post with 3-inch screws using a drill. Flip the post over, and do the same thing again with another 2-by-4 board. There should now be two boards parallel to each other at one end forming a T. Repeat the process with the other post and two boards.
Dig two holes three feet apart on center. This means that the measurement is from the center of one hole to the center of the other. Dig them 2 1/2 feet deep. Then, pour in 6 inches of gravel for ground drainage, and tamp it down.
Place the posts in the ground so that the T parts of both are parallel to each other. Fill the holes with layers of dirt and gravel, tamping each one down before starting the next. Use about 3 inches of material for each layer.
Place the 4-foot-long 1-by-6 boards across the T posts. They will hang over on each end about 5 or 6 inches. Leave a small gap in between them to allow rain water to drain. Drill screws through the boards and into the 2-by-4 T cross pieces. Place at least two screws in each end of each board.