Once your Ubuntu template is installed, you should be able to select it as the TemplateVM when creating new ubuntu-based AppVMs (persistent home directory, root filesystem based on read-only Template), or StandaloneVMs (separate, complete, VM with persistent, writable root filesystem and home directory). Depending on what software you will be running on Ubuntu, you may prefer to use a StandaloneVM instead of an AppVM.
To create an ubuntu-based qube, open the “Create qube” menu, and select “focal” from the template drop-down menu, and click create. Since ubuntu comes pre-installed with terminal emulators (i.e. xterm, gnome-terminal), these should already appear in the Qubes menu, which gives shortcuts to applications in each VM.
Open the Qubes menu and select your focal AppVM (or Standalone). There should be a pop-out menu with “Terminal” or “xterm” in the list, as well as a browser (probably Firefox). Click on any of these items to launch that application in your VM.
If you need to install additional terminals or browsers, or other applications, there should be clear info in the docs.
Hope this helps