It's really easy though, first download the ZIP file from Itch.io then extract it on a folder, after that, download both Joiplay and Joiplay's Ren'Py plugin from the Google Play Store.
After doing all of that, launch Joiplay, then click the (+) button, and fill in the necessary information (Game icon is optional), after that, locate the EXE, PY or SH file when you press the "CHOOSE" button on the "Executable File" field.
If you've done these steps correctly, Joiplay will automatically add the game to the menu.
Note: Joiplay also supports GameMaker Studio (Install the GMS plugin), Flash (SWF Files) and HTML games.
Enjoy!