1. Navigate in your Home Assistant frontend to Settings -> Add-ons -> Add-on Store.
2. In the ... menu at the top right corner click Repositories, add https://github.com/lmagyar/homeassistant-addon-mariadb-inmemory as repository.
3. Find the "In-memory MariaDB" add-on and click it. If it doesn't show up, wait until HA refreshes the information about the add-on, or click Check for updates in the ... menu at the top right corner.
4. Click on the "INSTALL" button.

For more details, please see the add-on's Info and Documentation pages.