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.