Slack-lining is a blast! This is very simular to tight rope walking.
1. Look down line (not at your feet)
2. Put your arms out
3. Bend your knees a little.
4. Bare feet is easier
This is important, you need a strong rope or straps to wrap around the tree, then hook your strap to the rope o strap on the tree.
Use the rachet to tighten the strap pretty much as tight as you can get it.
I like to use a 2 inch strap.
(you can go to a hardwear store and buy a tie down it will be much cheaper)