I recently saw a video on youtube, by Sean Mulhall (aka silver fox bushcraft) of making a birch bark hip flask. It's a good video and Sean is great at this kind of stuff, so I decided to give it a go.
My video isn't as good as his, but I'm quite pleased with the end result.
The video is awaiting approval, as it is a video response, but I will post it up ASAP.
Here's Seans video anyway.
Part 1
Part 2