Summarize this article in one sentence.
Position the loop of webbing about 2 to 3 ft (0.61 to 0.91 m) off the ground for a 15 to 20 ft (4.6 to 6.1 m) slackline. Loop the webbing tightly around the tree, then connect the ends with a carabiner.  Before using trees for anchors, cover the bark with padding like a towel or carpeting. If you’re creating a longer slackline, you may need to raise the slings up further to keep the line off the ground. If the line touches the ground when you climb onto it, take it apart and raise the webbing. Push the loop through a line locker to keep the slackline secure later. Then, pull about 5 in (13 cm) of the line back around and through the line locker again to create a double loop. Clip a single carabiner to it. Pull on the line to make sure the sling feels secure in the loop. If it feels loose, take it out of the line locker and carabiner, then knot it again. Lay the slackline out on the ground between the anchor points. Walk towards the loose end of the line and the second anchor point. Estimate when 80% of the line is behind you, then pull the line through a line locker. Tie the end of the line to a carabiner using a double loop like you did earlier.  You can usually get a good estimate of where to place the line locker by looking at the line as you walk. If you need to, measure it out with a ruler or tape measure. For example, if you’re using a 20 ft (6.1 m) line, place the line locker 15 to 16 ft (4.6 to 4.9 m) along the slackline. Place the slings on the anchor point closest to the free end of the slackline. Keep the slings close together. Connect the ends of each sling with a single carabiner. Lay the carabiners on top of one another, positioning them so the “gate” parts that open face in opposite directions.  Remember to protect trees by placing padding underneath the slings. Test the slings when you’re done to make sure they are knotted tightly in place. Bring the slackline underneath and through the lower carabiner on your anchor point. Then, pull the line back towards you and through the carabiner with the line locker. Pass the line under and through the upper carabiner on the anchor point. Finally, pull the line underneath and back through the carabiner with the line locker.  This is called a “primitive” 4-carabiner pulley system. It can be a little tricky to get at first, but it’s a safe, fun way to create a slackline without any extra tools or knots. If you have a ratchet system, attach the ratchet to an anchor point, then feed the slackline webbing through it.

Summary:
Secure a sling to the first anchor point at waist height. Hook the slackline to the sling with a carabiner Fold the first 12 in (30 cm) of the line over on itself to create a small loop. Place a line locker and carabiner about 80% of the way from the second anchor. Wrap 2 slings with carabiners around the far anchor point. Loop the slackline around the carabiners to anchor it in place.