This size is great for kids but a little snug for an adult. To make the swing bigger, even a two seat-er, just be sure to brace it well and to check the maximum load capacity for all parts.
Hardware: Braided nylon rope 3/8", enough to reach from limb to ground twice plus 10' (I purchased brown, designed for hunting, but plain white or yellow would be cheaper); about 10' of twine; (2) lag screw eye bolts about 4.5" in length; (2) 5/16" quick link connectors or "S" links (these will allow for easy removal and installation each winter); wood screws, 1.5" long, I used leftover decking screws designed for use with cedar.
Wood: redwood or cedar are great outdoor choices. I used a scrap piece of cedar leftover from the fence project about 17" long, 1" thick and 8" wide. I also ripped a second piece into two 4" wide strips about 6" long.
Happy kids: Priceless
First cut the braces as described above. Use a hand planer to round off all the corners and then sanded everything down with 60 grit paper. (One side was rough finished so I made it the bottom but still tried to smooth it out.) Then glue the two braces to the bottom of the seat, clamping it in place and countersink a few screws. Once the braces are secure, drill out holes for the ropes. It was easier to use a small bit (1/8") from the bottom and then use a 1/2" bit from the top using the smaller holes as pilots. That way the splintering occurred on the bottom rather than on the top of the seat.
Cut two 4' lengths of rope. Use a match or lighter to melt the ends of the nylon rope or else it will unravel. Thread each end through the seat, granny knot it and secure the loose end with a fence staple or bent nail. Find the middle of each side, loop and secure by "whipping" a 2' piece of twine. This forms the connecting point between the seat and main ropes and keeps the seat more stable.
Take an electric drill, the eye screws and a screwdriver up a ladder to the branch that the swing will hang from. (An easy method is just to tie the rope around the branch but it may rub off the bark, hurting the tree or the friction could fray and weaken the ropes. Be sure to select a sturdy branch that isn't too close to the trunk. Ours is about 8" in diameter and runs parallel to the ground.) Drill two pilot holes for the eye screws a few inches farther apart than the seat's width on the underside of the branch. This will help reduce twisting while swinging. Install the lag screws using the screwdriver to tighten them by inserting it through the eye.
Next, tie one end of the rope to the connector or "S" link using a bowline knot and attach it to the eye screw.
Then cut the rope to length, about one foot from the ground. Repeat with the other side. (This method will allow for uneven branches but you may want to mark which side is closest to the trunk so that it won't be rehung backwards each spring.) Next comes the tricky part, you may want to have the kids around for a custom fit but they may be too excited to let you get any work done.
Tie the hanging ends to the loops on the seat using a Beckett bend knot and whip the ends with twine. Be sure to get the seat level. This may take some trial and error. The tallest kid should be able to touch the ground with the balls of their feet but not their heels. It's helpful if they can push the younger ones too. This will be a very strong knot but still allow to be loosened later when you need to raise the seat.
Once all the connections are secure, set your daredevil down and let 'em fly. One maintenance tip would be to grease the connection between the links under the limb. It reduces noise, friction and provides a smoother ride. Enjoy!