This is how I make my shock cord tensioners. It's a little different method than Youngblood's but it achieves the same results.
Start out with approx. 12" of shock cord and approx. 5 1/2 feet of guy line:
Tie a bow line knot
in the end of the cord. This is what should attach the guy lines to your tarp. About 12" form the bow line also add a loop in the guy line:
Next take the shock cord through the loop in the guy line and tie a double fisherman's knot
to form a loop:
To finish, stretch the shock cord loop toward the bow line end of the guy line. Once to have found the maximum stretch of the shock cord loop, tie it off to the guy line at this point. This will create a limiter that will keep the shock cord from breaking:
A set of 4 of these shock cord tensioners (about 5' long each) weighs about .9 oz.