Install Mumble for desktop or for Android (Google Play, F-Droid).
tl;dr open this link
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 preferred username.
Here are the addresses my and my friends’ Mumble servers (you can click links to connect automatically):
After adding the server, press “Connect” in the “Connect” dialog.