Drive one stake into the ground to represent one corner of your swing set. Measure 12 feet and drive a second stake into the ground to mark the opposite end of the swing set. Dig post holes in each location. The holes should be 12- to 14-inches across and 24-inches deep.
Lay two 8-foot long, 6-inch-by-6-inch posts on the ground--parallel to each other--with the base of each post hanging over the edge of one of the holes. Install hurricane tie brackets at the opposite end of the posts, away from the holes. Use 3-inch treated deck screws to attach the brackets to the posts. Install one bracket aligned with the front and back face of each post. Turn the posts so that the front face of each post is facing up.
Lay a 12-foot long, 6-inch-by-6-inch board across the tops of the 8-foot posts. Position it so that the brackets cradle it on top of each post. Attach this 12-foot long board to the brackets with 3-inch treated deck screws in the same way you attached the brackets to the posts.
Measure and mark the location for each swing chain on the face of the 12-foot long, 6-by-6 board. Mark the side of the board that is facing toward the holes, this will be the bottom edge. Drill a ¼-inch pilot hole in each location and drive the eye-bolt portion of the swing swivel into each hole. Thread the bolts in, and turn them tight with a pipe wrench.
Have a helper assist you in standing the swing assembly upright in the holes. Attach a 2-inch-by-4-inch board to the outside of each of the upright posts with a single 3-inch treated deck screw. Angle it down to the ground to brace the frame. Angle the left one to the rear of the swing set and the right one to the front. Use a level to check the uprights and adjust as needed to center the bubble in the indicator. Drive a second screw through each brace board to hold it steady.
Fill each hole with dry concrete mix until it is 2 inches from the top of the hole. Add water to each hole, and mix the concrete. Fill the top of the hole with back fill dirt, and pack it firmly to help hold the swing set steady as the concrete hardens. Allow the concrete to set up overnight.
Remove the 2-by-4 braces from the uprights. Snap the clips of the swing swivels into the eye bolts on the bottom side of the top cross beam. Attach the chains of your swings to the swivels. Prefab swing chains typically snap in place with carabiner-style spring clips. For older style chains, use a threaded chain link of the same weight and thickness as your swing chain. Fit the link onto the swing chain and into the eye on the swivel, then tighten the threaded nut with pliers.