To start things off, you have to register your mobile number to your Google account. You can register this by attempting to buy on Google Play or in Apps with in-app purchases. If you haven’t previously linked a credit card, you will be given an option to “Enable Sun Billing”. After selecting this, you will be asked to enter your billing details such as your phone number, name and address. After confirming your details, you will now see “Bill My Sun account”. You are now ready to buy!