Lay one of the 4x4s on an elevated surface. Using your tri-square, mark a staight line at 8 feet. Cut it with your circular saw. Set the 8-foot piece aside.
Lay two of the 10-foot 4x4's on the ground to form the top two lines of a triangle. They should intersect at the top, with the right piece lying over the left one, and about one inch of overhang on each side past the point where they intersect. The bottom ends should be 10 feet from one another.
Using your level and pencil, mark a line on the right piece (the piece lying on top), with the line going from the lowest point of the intersection to the highest. Then mark another line from left to right, dissecting the two sides of the intersection. You should end up with what looks like a cock-eyed cross marked on the lumber, close to the top.
Set the lumber on an elevated surface. With your circular saw cut the vertical line of the cross, letting the short left piece fall. Then cut the horizontal line, letting the short top piece fall.
Flip the cut 4X4 over and lay it on lengthwise on top of the second 10-foot 4X4, with the two uncut ends lined up. With your pencil, mark on the top part of the second 4X4, using the first one as a template, to create mirror image of the pattern. Use your circular saw to cut the pattern into the second 4X4.
Using your pencil and level, make two exact copies of your two cut pieces, lying each of them on one of the remaining two 10-foot 4X4's to mark the lines. Cut the other two pieces in the same manner as the first two.
Lay the original two pieces back in their triangular position, but instead of intersecting them at the top, separate them by about 4 inches. The vertical cut lines should be parallel with each other, and the horizontal cut lines should be at the same level with about a 4-inch space between them.
Have your helper stand the 8-foot 4X4 on end between the two cuts. The configuration should look like half a swingset, lying on its side. The flat top of the 8-foot piece should be even with the two horizontal cuts, so the three pieces will form a straight line all the way across when the structure is stood up.
Secure the three pieces together using your long wood screws and drill with screwdriver bit, driving them in a few inches from the tops of the 10-foot pieces. Put at least four screws in from each side.
Turn the structure over, so the uncut ends of the 10-foot pieces are standing on the ground. Prop the loose end of the 8-foot piece on top of your ladder (with the other side held up by your triangular frame). With your helper, prop up the other two side pieces, one at a time, onto the ladder and secure them to the cross piece in the same fashion as the before.
Use your circular saw to cut one of the 2x6 pieces in half. Standing under the swingset, have your helper hold one of the short 2x6 pieces horizontally against the upper part of one side, flat against the two 4x4s, with the narrow top of the 2x6 pressing against the bottom of the crosspiece.
With your pencil, mark where the 2x6 lays against the outside of the 4x4s on each side. Take down the piece, cut at the lines with your circular saw, put the piece back up, and screw it in place with you drill and screwdriver bit. Repeat the process for the other side.
With your tape measure, mark a vertical line four feet down from the crossbeam on one of the legs, on the outside of the swingset. Hold your level at that line and draw it across both the 4x4 legs. Have your helper hold one of the long 2x6 pieces up to that line. Mark on the 2x6 where it lands on the outside of the 4x4s on either side. Take down the 2x6, cut it at the lines, put it back up and screw it into place. Repeat the process for the other side.
Position the fully standing structure exactly where you want it in the yard. Using your spade, mark around the base of each of the four legs. With your helper, move the whole structure a few feet to the side. Dig out each of the holes you'd marked, going down about a foot deep and a foot wide for each one.
Put the structure back in place, with the four legs in the four holes. Stand on the ladder and take a level reading from the cross piece, then adjust the depth of the holes to get the cross piece level. Do this by having your helper lift up on whichever leg is too high, as you scoop some of the dirt out from under it. The top beam should be 7 feet from the ground.
Using the shovel, mix the first bag of concrete and water in the bucket to a thick, soupy consistency (generally, about a 2-to-1 ratio of dry concrete to water). Shovel it into one hole, packing it in around the 4x4 post and mounding the concrete up around the middle. Repeat for each of the other three legs, with the other three bags of concrete. Let the concrete dry for a day.
Mark where you want your tire swing to go, using your pencil to put the marks on the underside of the crosspiece, measuring for the center of the piece. Drill a hole for the eye-bolt anchor hardware. Hang the swings and enjoy.