Drill the 48-inch 2x4s into place starting at the back, where it's important to be sure that the first slat is connected to the back support. Leave a quarter-inch (6-millimeter) space between the slats, which you can create using a standard pencil as a spacer.
The back slats are attached the same way, but with a half-inch or 13-millimeter (Sharpie marker-sized) gap between the first back slat and the bench and quarter-inch spaces for the rest of the back.
Finished? Good. You've earned the right to take a seat.
Bates, Jay. "How to Build a Comfortable 2x4 Bench and Side Table." Jay's Custom Creations. June 29, 2013 (April 8, 2014) http://jayscustomcreations.com/2013/06/free-plans-2x4-outdoor-bench