Now that you have a working project you should use the Yesod book to learn more.
You can also use this scaffolded site to explore some concepts, and best practices.
This is an example of a form. Read the Forms chapter in the yesod book to learn more about them.
And last but not least, Testing. In test/Spec.hs you will find a test suite that performs tests on this page.
You can run your tests by doing: