I had a look around online for a tute, but couldn't find anything like what I wanted (plenty of hooded towels, but that was about it). So I decided to wing it. I used one of the dress patterns I have lying around to cut out a simple A-line dress shape and then traced around the hood part of a hoodie in their current size. Then I cut out the hood pieces about an inch bigger all the way around.
They came together really well, and the girls and I are pretty happy with them.
No more shivering while we wait for our swimming lessons to begin!