Cut the wooden beams and strips to the proper dimensions using your power saw. Lumber doesn't come standard at 75 or 39 inches long. If you're uncomfortable with this step, most home improvement centers will make the cuts for you on site.
Sand all surfaces of your lumber. Use a rough-grit paper for your first pass, then smooth it with a second pass using fine-grit paper. You should be able to run your thumb across the surface with no splinters or discomfort.
Set the two beams on their 2-inch edges, parallel and with their ends aligned. Their outer edges should be 39 inches apart.
Set one of the wooden strips across the two beams. Its ends should line up with the outer edges of the beams. Its outside edge should line up with the ends of the beams.
Screw the strip in place with one screw at the centerline of each beam, driven through the face of the strip and into the edge of the beam.
Set the next strip in place, running parallel to the first strip and one inch away. You can use the narrow edge of a third strip to ensure proper spacing. Screw it in place as you did the first.
Continue adding strips as in Step 6 until you run out of strips. The final strip's outer edge should line up with the opposite ends of the beams.