1. In Google Classroom click the PLUS symbol.
2. Select Create Class
3. Enter a name for the class (optional: put your name in the Section field)
4. Click CREATE.
5. Click on the GEAR icon (top right)
6. Click the drop down for CLASS CODE and choose DISPLAY to show it on your projector or promethean board.
7. Instruct students to go to Google Classroom, Click Plus, and choose JOIN. They can enter the class code there to join your class.
See the video of me doing this: