To completely shut down the cluster, you simply need to turn both machines off.
To avoid potential disk corruption, ensure that you poweroff the STANDBY machine first.
That will immediately shut down the machine, and detach it from the cluster.
You can then shut down the Running node when it has detected that the standby machine is offline (you'll see it in Status)
To start them back up, if you turn them on at the same time, one machine will randomly start the cluster, otherwise the machine that started first will take ownership.
To avoid any data loss or corruption, leave the secondary machine turned off until FreePBX-HA is completely up on the primary node, and everything is running before turning it back on.