There are two ways: 

   1) When you log in to the game for the first time, connect your Facebook/Google account to log in.    

 When you come into the app, this is the first page you see,  

 then click "Login with Facebook/Google" button to link your account. 

   2) Lobby -> Settings    

 You can find "Setting" button here.  

 Once you touch the settings, you can link your account to Facebook/Google as below: