There’s a link to the test script in the instructions.
That test script should be placed right before the closing body tag in your code.
Click the green hamburger menu to expand it and then select the project whose tests you want to run.
If a test fails, click the red button to see which test(s) are failing and text to help you correct the issue. Be sure and read more than just the first line of the failing message. The ability to read and comprehend error messages is a skill you’ll need to acquire as a developer. Ask questions on what you don’t understand.
Edit: The test script, with all tests passing, should be included when you submit your projects.