Build a basic wooden swing set by connecting the uprights to the crossbeam and setting the structure in concrete. Once the frame is stable, attach the swing seats using eyebolts and nylon rope.
Determine the location for the swing set by placing the crossbeam on the ground. Adjust the beam until it is in the perfect location and mark each end using a wooden stake. Use marking paint to draw a 3-foot by 1-foot rectangle with the stakes at the center and the long sides perpendicular to the crossbeam. Dig each hole 18 inches deep.
Construct the frame using pressure-treated lumber. Sandwich the 2 x 6 crossbeam flat between the two 4 x 4 posts on each end. Use a square to ensure they align properly, and drill two holes through the 4 x 4s and 2 x 6 on each end. Attach the frame using 12-inch lag bolts.
With the help of one or more other people, set the frame upright and make sure it is level. Mix concrete and pour in the holes around the uprights. Once it is dry, cover the concrete with dirt.
Attach two sets of swing spring hardware sets to the crossbeam and tie heavy duty nylon rope to the springs. Leave ample rope to attach the swings approximately 24 inches above the ground level. Double knot the rope to attach the swing seats.