I made it out of plastic grocery sacks, which I turned into plarn, following the directions that I posted here. Once I had the plarn, I followed this pattern for a box-bottom tote at kaylaksthriftyways.com. I found the pattern easy to follow, and pretty enjoyable.
To be honest, the most time consuming part of it all was making the plarn, but it's a really handy bag and I love it. Although I'm sure it would make a great grocery bag, I use it for swimming mostly, I fold it up and put it inside a large canvas bag along with our suits, sunscreen, etc. Then when we're done swimming and have changed into our street clothes, I wrap the wet suits in a towel and toss them in this bag. It means that I leave the pool with two bags (my canvas bag and my plarn bag), but it also means that I don't end up getting everything in my canvas bag all wet from the suits. It's an awesome solution to what can often be a pesky problem.