In order to do that you have two ways at this moment, these are:

Apple:
It is automatically linked to your App Store account.

Android:
You’ll have to reach the configuration in-game.
At this point, you’ll see in the left corner a small joystick symbol, if it is red, you’ll have to click it. After doing that, you’ll be able to connect your account with the Google Play Store. After that, you’ll see the joystick symbol green and whenever you download the game on another device with your account already linked, you’ll be able to play in your account.