Cut the ends of each 128-inch board at a 30-degree angle with your miter saw. When the boards lie flat on their longest side, they should look like a trapezoid.
Position two 128-inch boards at a 60-degree angle. Two of the angled cuts should be against each other with the edges flush. This is the apex to your swing stand. You will need a large open and level area for this task. Screw four screws through each board and into the angled end of the board it is up against. Use eight screws. Repeat this step with the other two 128-inch boards.
Lay an 18-inch board underneath a pair of 128-inch boards so it is perpendicular to the joint on the apex and 15 ½ inches below the top of the apex. The top two corners of the 18-inch board that are closest to the apex should be flush with the outside edges of the 128-inch boards. Lay another 18-inch board on top of the pair of 128-inch boards so it is aligned with the 18-inch board underneath. Drill two ½-inch holes through each end of the 18-inch boards and through each 128-inch board. Space the holes 2 inches apart, and leave 1-inch of space from the holes to the edges of the 18-inch board. Bolt the boards together using 4 bolts. Repeat this process with the other pair of 128-inch boards.
Bolt a 78-inch board to each side of each pair of 128-inch boards. The boards should be parallel with the 18-inch boards and 68 inches below the top of the apex. The top corners of the boards should be flush with the outside edges of the 128-inch boards. Bolt the boards to the 128-inch boards in the same manner as the 18-inch boards were bolted. Each pair of 128-inch boards are the completed sides to your swing stand.
Position the sides upright, parallel and with the outside edges of the 2-by-4 boards 144 inches apart. Insert the 144-inch board through the hole on each side that is between the 18-inch boards and the apex. Drill four 2-inch deep, 3/8-inch pilot holes through each 128-inch board. Screw through the 128-inch boards and into the 144-inch board. Use 16 screws for this step. Attach your swings to the 144-inch board using the instructions and hardware provided.