First, go to ["My Live Events] on YouTube. Creating a new live event, or, if you want to record now, simply "Go Live Now".
Fill in the information for your event, and press "Create Event". Make sure to choose "Unlisted" if you don't want it publicly available. It will still be accessible to anyone with the link.
At the schedule time, go back to ["My Live Events"], press "Start Hangout on Air"