If you have a hand ascender even better. Stretch climbing rope between two trees. Run a climbing sling and carabiner from your bridge to the rope as a backup. Clip your hand ascender on to the rope, and to a carabiner on your bridge. Get some good gloves. Lean back and pull yourself head first to the other side. Easy as pie. Can also be done with cables for zip line, but it gets tricky getting tension set right to go both directions easily. With rope and progress capture you can have the rope set at same height on either end and use your arms to pull yourself forward.