Test-driven development (TDD) flips the usual order:

Red: write a test for the next small thing the code should do. Run it; it fails, because the code doesn't do it yet.
Green: write the simplest code that makes the test pass.
Refactor: tidy the code, knowing the tests will catch anything you break.

Then repeat with the next behaviour. It sounds slow, but it keeps every step small, tells you exactly when you're finished, and leaves you with a full set of tests for free.

You've been doing half of this all course: each exercise's tests are written before you write the code, and Check turns red to green.