I used this GitHub plugin to connect to Google Play Services and it seems to work:
My next step in development was to save and load players' progress (once the player completes a level, the game would save, and then once the player reopens the app, the data is loaded).
I've been working on this for weeks, looking at tutorials, etc. but I can't find a solution to saving and loading the player's data.
Does anyone know a good tutorial or can explain in detail what I should do in this situation?