You can tap on your Coins at the top left corner of the screen for quick access. Alternatively, you can tap the Menu button at the top right corner of the screen and select Shop.
Select the desired amount.
The checkout page will appear. Please click on the Payment method to display all options and then select Redeem Code.
Select the payment method and choose "Redeem Code". Enter the Google Play voucher code and click on the Redeem Button.