Build your zip line system with safety as your first priority. You can use cable slings that wrap around the trees to attach the zip line, but the safest way is to drill through the tree and use forged eye bolts to secure the zip line cable. There are many kits available with special hardware to ensure proper installation and safe operation of your zip line. You may want to check with the zoning laws in your area and see if a permit and/or inspection will be needed, regarding safety or code.
Drill a hole through the trunk of the tree from where you will launch on the zip line. Remove just enough bark around the hole on both sides to accommodate the washers.
Slide a washer onto the eye bolt and insert the bolt into the hole through the tree. Add another washer and a nut at the other end of the bolt and tighten. Repeat these steps for the other tree where you will attach the end of your zip line.
Make an eye on the "launch" end of the cable by using a cable thimble. Wrap the cable around the thimble, allowing a 1-foot length of cable past the thimble. Repeat this process for the opposite end of the cable.
Place clamps to secure the cable to the 1-foot excess slack and tighten. Use three clamps, placing one close to the thimble to keep the cable secure on the thimble and two more spaced evenly along the 1-foot slack.
Unscrew the pin on one end of one of the turnbuckles. Slide the pin through the thimble on the prepared end of the cable. Tighten the pin.
Attach the turnbuckle to the eye bolt on the "launch" tree. Unscrew the pin on the other end of the turnbuckle, insert it through the eye bolt and secure it by screwing the pin tightly.
Attach a turnbuckle to the eye bolt on the second tree. Unscrew the pin on the other end of the turnbuckle and insert the second thimble. Tighten the pin securely.
Place the wire through the cable end of the turnbuckle, on the thimble. Place one clamp, barely tightened, to hold the cable in place.
Tighten the cable using the pulling kit. Secure the cable with the three remaining stainless steel cable clamps and tighten.
Test your cable for excessive sag and use the pulling kit to tighten more if necessary. Check all bolts, clamps and pins on the turnbuckles to ensure that all are tightened securely. Trim away any excess cable using cable cutters.
You may want to install a braking system on your zip line for safety. Always have another person present when you use your zip line. Keep the area under your zip line free of debris, such as dead fallen branches, small trees or rocks. Always wear a helmet and gloves when using the zip line, and keep long hair secure with a hat so it won't become caught in the harness pulley attached to the cable.
Never leave a harness attached to a zip line unattended. This will prevent anyone from being tempted to take an unauthorised ride. Perform a complete visual check of you zip line prior to each use. Check and tighten all nuts, pins and clamps periodically. Never ride the line if the cable is frayed.