How to Set Up a Slackline

Setting up a slackline is pretty easy.  Unless you're going for one of those Gibbon gucci slacklines with the thick webbing and ratcheting come-along, your entire setup can be purchased for $50 or $60 (probably less in the USA).

Equipment 

Here's what you need (clockwise in above photo):

• 1" Tubular webbing - The rolled, blue tubular webbing above is 15 m of climbing-strength webbing, rated for 17 kN.  MEC sells it for $1.30/meter here.  This is obviously the main part of your setup and should be good quality.  15 - 20 m should be good.
• 1" flat webbing (anchors) - Above I have 2x rolls of 4 m (8 m total) flat webbing.  You need something like this to wrap around your trees.  The larger the tree trunk, the longer the anchor must be.  The webbing above is accessory webbing and is not intended for climbing.  It's only rated for about 11-12 kN and is sold for $1.00/meter at MEC.  This should be fine, but if you want something of a higher grade, try MEC's flat climbing webbing.
• 4x oval carabiners - Do they have to be oval?  No, D-carabiners would work too.... BUT, Ds are more expensive to purchase and they don't hold webbing as nicely.  The Black Diamond biners above are $5.25 ea. at MEC and work great.
• 2x descending rings (optional) - Optional because they're not required for the setup, BUT they will ensure you do not need to tie knots in your webbing.  Knots weaken the webbing and distort its strength ratings.  Plus, I don't want to spend their time tying and untying knots.  MEC says they carry descending rings but my location does not stock them.  Wilderness Supply stepped up to bat and sold me the above rings (20 kN) for less than $4 ea.  Line lockers or padded chain links would work just as well.

Set Up

1. Find two trees (~ 25+ cm diameter) between which to set up your line.  Ensure you have about 5 m more webbing than distance.  
2. Wrap your anchors around the trees, several times if necessary (doubled if possible).  I sewed loops in both ends of my anchors for easy set up (if you do this, make sure you know how to sew the correct patterns for max breaking strength).  If no loops, use a water knot to connect the ends of the webbing together (see below)

3. Connect an oval carabiner to one anchor and fix your main line (tubular webbing) to it using the descending ring and a bight + line locker.  If you're not sure how to do this, check out these instructional steps.  If you didn't buy descending rings, you'll need to tie your main line to the carabiner.
4. Bring your main line to the other anchor (ensure no twists).  Add another carabiner + descending ring to the main line about 1 m from the anchor when taut, or about 20% of the total distance.
5. Steps 5 thru 8 will give a 3:1 mechanical advantage to tighten the slackline.  Add two carabiners to the anchor webbing and feed the main line through one of them.  If the top one, feed the line top to bottom.  If the bottom, feed it bottom to top.
6. Bring the main line back to the biner/descending ring combo and feed it through the carabiner the same way you did in step 5 (top to bottom or bottom to top).  
7. Bring the line back to the anchor and feed it through the other ring, the opposite way from step 5.
8. Feed the main line back to the the carabiner in step 6 and bring it under the webbing from step 6.  You should now have a tension lock and the line should hold itself up.  It should look something like this:

9. Finally, grab that loose end and pull!  Keep pulling until you tighten to the desired amount of 'slack'.  You may want to enlist the help of a friend.  The line is under a tension lock and will hold itself... BUT just to be safe, tie off that extra webbing in a knot as a backup.

Happy slacklining!