Step 1: Sign into your developer account console HERE
Step 2: Upon signing in with your credentials, you will see a dashboard listing your Apps.
Step 3: Click on the App for which you'd like to subscribe to a Paid Plan and choose the "App:Billing" tab.
Step 4: Click on the SUBSCRIBE button of your chosen plan and fill in your credit card details.
Step 5: After entering valid credit card details, click on the 'SUBSCRIBE' button and confirm your plan selection.
You should see a message confirming your subscription. A confirmation with a receipt will also be sent directly to the email address associated with your account.
Other articles you may be interested in: Can I pay by invoice instead of by credit card?| What are Peer Connections and how are they measured?| How is TURN Bandwidth measured and billed?| How is MCU/SFU Bandwidth measured and billed?| What happens when I exceed the included amount of Peer Connections or Bandwidth in my plan?