Slack lining is a very fun and challenging "sport" that requires a lot of skill. However the cheapest slack line "kits" available are around fifty bucks, i made this one for about half of that. This instructable will show you how to set up your own slackline.

Step 1: Supplies

Here is what you will need. I got all of the supplies from REI, but i am sure there are many other places that carry these things.

Materials
1 inch tubular webbing- (at least) 70 feet
carabiners - 5
carpet (1ft. by 3-5ft.)

also needed:
2 trees, poles, or any two VERY sturdy upright objects

Step 2: Anchors

The anchors attach the line to the tree or whatever you are slack-lining in between.

so, once you have the webbing, you need to cut of TWO ten foot sections (make sure to burn the ends, so the webbing won't fray). these will be the anchors.

Now, tie a water knot on both ends of the ten foot sections to make a big loop.

you can see how to tie a figure water Here

Step 3: Knots

... well hitches actually
there are only two simple knots that you need to know
these are the Clove, and half hitch

if you dont know how to tie these, look here:
Clove Hitch
Half Hitch

Step 4: Set It Up - Anchors

wrap the carpet around the tree, this will not only protect the tree, but make your line last longer.
then wrap one of your ten foot sections around the carpet and put one carabiner on the ends of the loop

do the same to the other side, but with two two carabiners on this side. make sure that BOTH carabiners go into both loops with the gates facing out

Step 5: Set It Up - Slack Line

now, tie one end of your large piece of webbing to the anchor with only one carabiner, use the clove hitch.

now walk towards your other anchor holding the line to make sure that there are no twists. when you are about 4/5 of the way there tie a clove hitch, put 2 carabiners through the two loops you make in the clove hitch.

Step 6: Set It Up - Slackline

now feed the exces Webbing (the free piece of webbing) through the bottom carabiner on your anchor, going up, then feed it back through the bottom carabiner on the line,going up, then the top carabiner on the anchor, going up', and the top carabiner on the line, going up.

now PULL, this might take more than one person. you want it to be pretty pull until it won't go anymore.

Keeping tension on the line, tie at least three half hitches around the webbing that is in between the sets of carabiners. I recommend tyeing them on a bite, this will make taking down the line much easier.

Step 7: Set It Up - Slackline

now, while keeping all the tension on the line tie 3 or 4 half hitches in front and in back of the lines's carabiners.

Step 8: SLACKLINE!!

now you're ready. this takes a ton of practice, but eventually you can get pretty good, have fun and be safe!!

Note that it will take a while for your line to stretch out, if you notice the line getting loser, it is probably the webbing stretching.