Cut a piece of 2-by-8-inch oak approximately 16 inches long. The oak board will become the seat of your swing. If you want to make a double swing so two people can swing at the same time, cut the wood to a length of 32 inches. This works well if the branches are a bit farther apart. Use a miter saw to cut the wood and then sand the edges with a sanding block to smooth the wood.
Drill a hole approximately 2 inches from each end through the 8-inch face of the wood. Make the holes large enough to fit your nylon rope. Use a nylon rope at least 1 ½-inches in diameter. To cut the hole, you will need a hole saw and a hand drill.
Paint the wood with a stain and sealer or with an exterior latex paint. Use a paintbrush or a sprayer for this. Allow adequate drying time per the instructions on the paint container. You can be as creative as you like with this step.
Measure from the center of one hole to the center of the next hole and transfer the measurement to the two branches on the trees. Drill down through the tree branches with a drill bit and a hand drill.
Place an eyebolt through each branch. Place a large washer onto the eyebolts and then secure them to the trees with a nut and a wrench.
Tie a piece of nylon rope to a rope thimble and then secure the thimble to the eyebolt. Do the same with a second piece of rope and secure that thimble to the second eyebolt.
Thread one rope through one hole in the swing. Tie a knot to hold the swing onto the rope. Thread the second rope though the second hole in the swing. Tie a knot in the rope to keep the swing in place. Make sure the swing is level.
Cut off the excess rope with a hacksaw.