Measure out a length of inch-wide polyester webbing that's 3 to 5 feet long and add 6 inches to your final measurement.
Cut two lengths of polyester webbing that match the dimension in step 1.
Fold down one end of one of the straps by 3 inches and sew it across the edge to make a loop at the end of the strap. Use a sewing machine and make several passes to ensure the loop is secure.
Repeat step 3 for all the other ends of the harness. When you're done, you have two strips of webbing with loops at either end.
Hook a carabiner through one loop on each strap. Use the harness by drawing the end of the strap without the carabiner around the tree and pulling the end with the carabiner through the loop, pulling it tight. The carabiner then hooks through the hammock.