Commercial slackline solutions seem bulky, heavy, overpriced, and a little too industrial for my liking. I have 6 carabiners and 50m of tubular webbing. What knots do you recommend for building, anchoring and tightening a slackline?
I am not (yet) a slackliner, but I have been impressed with the methodology of testing and development of Adam Burtle of NWslackline.org. His site has a number of videos that demonstrate different anchoring and tensioning systems. He also does line tension measurements and break testing on different setups and materials.
When you watch his videos make sure you have annotations turned on as they often contain important details, corrections or revisions.
I cannot recall seeing an anchor/tensioning system using only carabiners but the line-locker system using a forged rappel ring is simple, light and inexpensive. Chain links are even less expensive but need to be deburred to protect your webbing. One of his basic tensioning systems looks like this (the load cell is only for testing of course):
In Strength of 3 men, for only $20 (article and video) he rigs a basic pulley system:
More advanced equipment and techniques will be needed for long lines.
You have probably already seen the excellent resource at www.slackline.com but they have an excellent page on how to tie your slackline to a tree using ordinary webbing and carabiners.
Key image from their sequence:
Then just add a carabiner (with webbing wrapped round it to protect) to the metal ring and you are set.
Total kit required:
For tensioning, people use 3:1 or even 6:1 pulley systems with some success, but you'll find it hard to get enough tension to avoid bottoming out.
This guy tries a few pulley setups, but still has a fair amount of drop in his line.
Realistically, you'll want a ratchet like this one - it's the only way to get enough tension in your line!