After you finish your work, click on the circle icon that will show on the right side of the blue top bar (you can also go through My timesheets page in the menu).
First, you review the case (prepared for you when you e.g. edit a document).
You can change the person or date.
Then, you fill in the time in.
You can choose from a list of activities and write a description that will be ultimately shown to the client in his invoice attachment.