You can install Mumble from https://mumble.info. After you've installed and launched the desktop client click the "Server" button at the upper left:
Now choose "Connect" from the dropdown:
The "Connect" dialog appears. Now click "Add New...":
There you can fill up the details of the Mumble server.
You only need to specify the address and your preffered username.
Here are the addresses my and my friends' Mumble servers:
- commandandconquer.club - my server
- britbongistan.club - Bricky's server
- jihadi.group - Hector's server
After adding the server, press "Connect" in the "Connect" dialog.
TODO: describe the mic config