Enter your in-app profile and press "Get Premium subscription" to buy your subscription.
After that press "Try Premium" to get a free-trial that will be automatically upgraded to paid premium subscription later.
Paid subscription automatically upgrades after the trial period and also it is automatically updated every month. You will get all the information to your Google Play linked e-mail.
Beware that you are not able to make a refund for a subscription, you already paid for. You can read more about it here.