Prepare the legs. Use 1 1/2-inch by 3-inch softwood, treated with a preservative such as polyurethane. You will need eight pieces cut 27 inches long.
Prepare the cross-bars. Cut four pieces of 1-by-2-inch softwood. Allow 3 inches of overhang on each end.
Cut the slats to the required length. For this example, cut them to 60 inches or 5 feet long. Doing so will allow more people to sit or one person to lie down on the bench on a lazy afternoon.
Lay the slats and crossbars flat on the ground and start assembling. Use carpenter's glue first, then use a clamp to keep them together.
Connect the legs to the cross-bars first, keeping the legs 15 to 18 inches apart from each other in two rows. Use galvanized screws or nails.
Connect the slats on the legs and cross-bars to complete the bench. Leave at least 1 inch of space between each slat. Use galvanized screws or nails.
Paint the slats and crossbars with white paint (or wood preservative or stain). Use an outdoor type of paint or finish to withstand the elements.
Allow the bench to dry before using. Apply more paint or wood stain until you get the desired look and finish.