Dig six holes with the posthole digger. Always wear work gloves when using a posthole digger to avoid blisters. Each hole should be 18 inches deep and 12 inches in diameter. Space the holes in pairs, leaving 6 feet between each pair of holes. The first pair of holes should be 7 feet from the second pair and the third pair should be 4 feet from second two. The dimensions of the rectangle made by the holes should be 6 feet by 11 feet.
Cut notches 4 1/2 inches deep in the tops of six of the 4-by-4-inch posts, using the circular saw. These notches will hold the 4-by-6-inch post in place as the top upper frame of the swing set. Each notch should be cut at the same point on each post.
Put two of the 4-by-4-inch posts in a pair of holes and lean them toward each other until they touch, forming a triangle at the top. Measure the distance between the posts at 10 inches down and 30 inches down from the peak of the triangle. Use the circular saw to cut 6 cross braces out of the seventh 4-by-4-inch post. Cut three braces to fit between the posts 10 inches down from the peak and three to fit 30 inches from the peak. Nail each cross brace in place between two of the 4-by-4-inch posts to form the A frames of the swing set.
Put the other four 4-by-4-inch posts into their respective holes, then bolt all the posts in place with the hardware brackets. Slide the 4-by-6-inch post in place between the notches you cut earlier, then bolt it in place as well. Fill each hole 3/4 of the way to the top with the concrete mix, then fill up the rest of the hole with water and allow the concrete to harden overnight.
Sand down all the posts with the power router until they are smooth. Drill holes in the 4-by-6-inch post for the swing set hooks. Drill holes through the cross braces into the posts they are attached to, then insert screws into these holes to better anchor the set. Screw the swing hooks into the holes you drilled for them in the 4-by-6-inch post, then attach the swings.