Just cut the tea towel in half, then sew the two pieces together into a big cylinder and sew over the raw edge at the top so that you can thread elastic through. Easy peasy.
They were really quick to do and the elastic was from the dollar store too, so they worked out at only $1.50 each. The kids have worn them all day and love the little pockets on them (which came from a mystery bag of sewing odds and ends from a garage sale). Here are my daughters being loony in them. Highly recommend giving these a shot, even if you're like me and are sewing pattern phobic ;) I want to get some more to make gifts and also try and make some bloomer/shorts, but that's getting dangerously close to requiring a pattern ;)