Alas, Google is moving Hangouts On Air from Google+ to YouTube Live on September 12. After that date, you will not be able to schedule Hangouts On Air on Google+. Already have one scheduled? You'll need to move it to YouTube Live.
At least you will still continue to have access to your recorded event on YouTube. Google+ will also continue to have your event content available in read-only format in the Activity Log.
If you want to broadcast an impromptu meeting with YouTube Live, you'll need to take the following steps:
Either way, you can control who sees your broadcast:
Public - Anyone can see and access the event.
Unlisted - The event isn't shown publicly but anyone with a link can join. This is identical to the previous Google+ Hangouts On Air setting.
Private - Only people you specifically invite can join the event. You can share the event with a domain, group, or individual.
In addition, you'll need to set up a local live-streaming encoder. This alone makes streaming with YouTube Live much more complicated.
More advanced Google+ Hangouts On Air functions will no longer be supported. These include Q&A, Showcase, and Applause.
I find this a disappointing move. Besides being a die-hard Google+ user, the Google+ version of Hangouts on Air was more interactive with viewers than the YouTube version. Oh well. It's gone now. I hope that Google will add these missing features to Google On Air with YouTube Live soon.