1. Create a file /etc/apt/sources.list.d/onsala.list with the following two lines:

   deb http://nain.oso.chalmers.se/oso/ubuntu/ jammy contrib
   deb-src http://nain.oso.chalmers.se/oso/ubuntu/ jammy contrib
   

2. To add this repository to your computer, download onsala-archive-keyring.deb in this directory, then do:

   sudo dpkg -i /path/to/onsala-archive-keyring.deb
   sudo apt update
   

3. Now update your package cache and install the application you want, e.g.:

   $ sudo apt-get update
   $ sudo apt-get install xs
   

Obviously you can also handle this via the Synaptic Package Manager or other software update tools.

This should work for Ubuntu 22.04 (jammy) and Debian 12 (bookworm), as well as other distributions based on these (like Mint 21, LMDE 6 etc).