In the meeting menu, click Share. Your alternatives are Screen, Whiteboard, iPhone/iPad or any of the programmes open on your computer. The difference between Screen and a specific programme is that if you choose Screen, the participants will see everything you do on your screen and will follow along when you change programmes, whereas if you choose a specific programme, the participants will only ever see that one, no matter where you go on your screen. At the bottom, there are also two boxes to tick: Share Computer Sound and Optimize for full screen video clip. Use these if you want to show a film through your shared screen.