You don't practice by doing easy things you already know.
Take something new and just start building... even if it's something stupid.
IE: I took used Silex (php micro framework) and built my own RESTful API for my personal website. It was so pointless and unnecessary but taught me a lot. It was just something simple that could've been hardcoded into the HTML but was fun to optimize and improve on every few months.
For example instead of hardcoding my name in the HTML I used a GET /name and instead of a typical contact me form I used a POST /contact etc etc. Very very very basic endpoints that aren't necessary but teach you the basics of designing an app.