Pages 25-27 of the User's Manual detail the steps, but it is pretty easy.

1. Connect the microscope screen to your classroom PC's USB port

2. Select DOWN (V) and OK on the microscope to put it in camera mode

3. Start the webcam app on your PC, probably XSight HD on your desktop.  This will display the microscope view to your PC.

4. Turn on the overhead projector to display your PC screen.