From the main Time Tracking page, click the purple play button in the top left of the screen to begin the timer. The clock will begin counting up from zero.
To pause the timer, click the red pause button.
To attach tracked time to a project, click the timestamp (XX:XX:XX format) next to the purple play button. This will open the active timer menu, where you can add the time to a specific project, specify your hourly rate, or add notes about what you spent your time on.