ImmortalCord157 has already written a comment with all steps, which is very short & to the point. Great for those who want fast instructions. I made some more in-depth instructions for those who might need more details. :) 

Step 1: Download the the JoiPlay app (.apk file)

Joiplay is not on google play, download the most up to date version (free) from joiplays patreon page: https://www.patreon.com/joiplay

Latest version as of 2024-03-05: JoiPlay 1.20.32, https://mega.nz/file/Hr5XRSgS#IU-1rw7aBsgOqlTmPkdVCmodTqXRN7P9L1Az-YY5sLA

Step 2: Download the RPG plugin (needed to play 'small on top')

On the same joiplay patreon page, download the latest RPG Maker plugin. This allows joiplay to start games created with 'RPG maker'. https://www.patreon.com/joiplay

Latest version as of 2024-03-05: RPG Maker Plugin 1.20.53, https://mega.nz/file/OqgF3AaY#P0t6Rwtu1wobHbqotd7tAWbOXTKE-xMj0I5bbqVVMsw

Step 3: Install the .apk files

You will have two .apk files from the previous steps in your download folder.

Android application .apk files are similar to windows .exe files. Open your downloads folder (using the 'files' app or chrome downloads), and start the file called joiplay[some_version].apk file

If you have not installed an .apk file outside google play before, your device might warn you about opening unknown files. Press 'enable installing from unknown sources'

When JoiPlay is installed, install the RPGMPlugin-[some_version].apk download in step 2 the same way.

Step 4: Download 'Small on top [version].zip' from itch / patreon

Open your downloads folder, select the 'small on top.zip" and select "Extract". (Will load a a couple of seconds)

Step 5: Install 'small on top' in Joiplay (aka press "ok" on a lot of prompts)

Launch the JoiPlay app, and accept the Terms & Conditions & privacy policy

If you have android 13+, JoiPlay asks for permission for notifications. It's not required to accept.

When it asks for storage permissions, press "Allow" on all 4 prompts. (If the first prompt appears again, thats a bug, close the application (recents menu -> swipe away), start joiplay again and press 'ok' again.) A window will appear asking for "All files access". Find "JoiPlay" in the list and turn the toggle on, and then press back.

When the permissions are granted, you see a white screen with a plus in the upper right corner, press that, then "Add Game" -> Choose  -> /Download/ -> Small on Top -> Small on Top (again) -> Small on Top (again again) -> Game.exe . 

Joiplay will ask what you want the game name and version name to show up as inside Joiplay, go with the default "Small on top", and whatever you want to call the version you downloaded

Skip adding an icon for the game, use the default icon & press next.

Step 6: Play the game.

Press the icon "Small on top". The game will now finally start.

Press the small arrow icon at the bottom of the screen to get on-screen buttons you can use in the game to move / press the menu.