You will need a regular version of Aseprite (not LibreSprite, not the trial version - these two don’t support extensions as far as I know).
You then download the extension file and open it with Aseprite, you should then be prompted to confirm if you want to install the extensions.
After that it should install the extension without any additional steps, and you should find the new options/features available immediately (sometimes you need to restart Aseprite for everything to work correctly but that’s rare). ✌️