In order to link your current account to Google Play you will need to open your in-game Settings Menu.
You can find your Settings by tapping on the gear icon on the left side of your in-game screen.
In the Settings Menu, you will find the option ''Sign-In with Google Play''.
Simply tap on the button and your account should then link to your Google Play account.

If your Google Play Credentials are already linked to another account, an error message will appear, telling you that the action cannot be completed.
In case this happens, please contact Support.