You can easily share your screen during a session,
Step 1: To share your screen with other users, click on the screen icon in the lower right corner.
Thereafter, your browser will open a pop-up window where you can define the content of your screen that you want to share. (1)
Step 2: As soon as you allow the screen sharing (2), the selected content will be displayed to all participants in the main window.
Step 3: To stop sharing your screen, either click the "Stop Sharing" button in your stream (3) or alternatively click the screen icon again. (4)
For being able to collaborate and annotate the content, you must either upload the respective file (see "How can I upload files to collaborate?") or take a snapshot and start the collaboration.