You can create a new activity in the Calendar tab, just click the + Add Activity button, give your activity a name and select the colour you'd like to associate with this activity and then click the Create button.
You will see a list of all the activities you have created on the Calendar tab. Simply click the "play" button next to the activity that you want to start tracking. The timer will appear and start tracking.
To stop, just hit the "stop" button either in the column with your other activities or on the tracking time entry itself.
(Please note that the activity needs to last for at least 1 minute in order to be saved.)
Now that you know how to create activities and start tracking, have a look at how to edit or delete your time entries, how you can create subprojects by adding notes or filter and export your data.
Happy tracking! Any questions? Don't hesitate to reach out.