There are several places online where you can learn how to cut up t-shirts to get one continuous 'strand' of 'yarn'. You single crochet like crazy and then start with a new one when you run out of 'yarn'.
I'm not done yet, but here's my progress to date. I can't begin to tell you how fun this is! And it's super comfy under-foot. Can't wait to have this as a throw rug on the bedroom floor at the cottage!