CAUTION! This method is not to be used as a highline! You will probably only get four or five uses out of the tape before it begins to fray. Once that happens you should replace the tape. This method should only be used to see if you like slacklining enough to invest in good equipment.
Here's what you'll need.
2 ea. welded steel rings (75¢ ea.)
2 ea. welded "D" rings (75¢ ea.)
1 ea. Steel "S" hook ($1.99)
100' of polyester wire pulling tape. (5¢-10¢ per foot, but most electricians use it once and throw it away, so if you're resourceful you can get it for free)