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 to confirm your plan.
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.