Here's a step by step guide to help you:
Step 1: Go to Google Play Store and select the app you want to download from the list.
Step 2: Click the price button to be redirected to the page for App permissions. This portion includes storage requirements and other details. You need to "ACCEPT" the app permissions to proceed to the next page.
Step 3: After clicking the "ACCEPT" button, you'll see the name of the app that you want to purchase along with the price button. Click "Continue'.
Step 4: That leads you to the Payment Options. Select "Mobile Billing" or "Globe Billing" to pay your app via Direct Carrier Billing.
Step 5: An account verification page will be displayed next. Once the account verification process is complete, you'll be directed to the registration process. Registration process happens only once during your first purchase. Enter your name, phone number, and billing address.
Step 6: Accept the ‘Terms of Service'. That takes you to the page where you see the app name along with the price details that you wants to purchase. The price is exclusive of VAT charge. You'll be charged 12% VAT over and above the displayed price. Click on the "BUY" button to proceed with the purchase.
Step 7: Enter your password and that's it!
For more information, you may refer to this link: Direct Carrier Billing for Google Play Store