It's easy!
First, check out the issues on the GitHub page. Once you have a feature, or bug in mind, fork this repo, clone your fork and create a branch.
The branch you create should have a name like this:
{some branch title}-{issue number}
ex: coffeed-12
Make your modifications and submit a pull request!
Thanks for contributing to the HyperTextCoffeePot!
Please look through the issues list and feel free to ask questions! A lot of the issues can appear terse, but its probably because they're 'notes to our future-selves'. If a feature looks interesting to you, but you're not clear on the details, ask away!
Flask: for the web server. ... I think that's it.