It’s quite simple and straightforward! You can save your game progress with either Apple ID or a Facebook account.

To save your game progress with Apple ID, do the following:
  1. Open Game Options (the gear icon);
  2. Tap  "Save progress";
  3. Tap "Sign in with Apple".
  4. Done! Now your game is linked to your Apple ID, and your game progress is safe.
To save your game progress with Facebook, do the following:
  1. Open Game Options (the gear icon);
  2. Tap  "Save progress";
  3. Tap "Sign in with Facebook".
  4. Done! Now your game is linked to your Facebook account, and your game progress is safe.