The title bar on each exercise page is red. Read the lesson and the task thoroughly. Write the solution into the text field and start the tests. If you made a mistake, a test is red and the title bar remains red. The red test tells you what you did wrong. Correct the error and restart the tests. If all tests are green, you will receive a success message and the title bar will turn green.
Use the arrow keys in the title bar ( < # > ) to go back and forth between the exercises. With the hash you get to the overview. Here you can find all exercises.