You can only share your screen when you have the Floor.
The "Share your screen" icon can be found on your lower screen.
Once you have clicked "Share your screen", the browser will ask you to confirm what exactly you would like to share (example from Chrome):
If you choose screen sharing from the "Chrome Tab", choose the tab you would like to screen share from. For screen sharing with audio, you need to checkmark ‘Share tab audio’ in the left bottom corner.
Please make sure to choose an appropriate screen and click “Share”.
If you wish to stop screen sharing, please click the pop-up message "Stop sharing" on your shared screen.