It's the same way for all .apk files you find on itch.io:
1. Get the Sidequest Desktop App:
https://sidequestvr.com/setup-howto
2. You'll need to watch the tutorial video that is linked to on the download section for the Sidequest Desktop App and follow the steps there.
3. Download the .apk file for the game and install it through Sidequest onto your Quest 3S.
4. You'll be able to find the game in the Unknown Sources section of the Library app after you've installed it.