...
Whether your breakout room assignments were preplanned (as in Method 2 above) or they have yet to be assigned (as in Method 1 above), the way you launch Breakout rooms is the same - through the Breakout Rooms tool 
in the meeting control toolbar during a meeting.
...
Once all the particpants leave for their own breakout rooms, the host will be left alone in the main room. Clicking on the Breakout rooms tool in the meeting control toolbar will bring up a list of all the active breakout rooms. Next to each breakout room there will be a blue join link, which will allow you to join any particular breakout room. Also in the Breakout Rooms - In Progress window, there will be a Broadcast a message to all button and a Close All Rooms button. Note that the way to communicate with everyone when they are in their respective breakout rooms is to use the Broadcast a message to all button and NOT the Chat button.
The Breakout Rooms - In Progress window will look like this (note the Broadcas a message to all and Join links):
Some potential failures with breakout rooms:
...