On the website, go to the home screen and click on My Subscriptions in the left side menu. It will reveal a Your Cards link to add a credit card to your account. Once added, you'll easily be able to signup to any content creator subscriptions from their profile using the Offiicial Follow buttons.
From the mobile app, open the side menu using the Hamburger icon and click on Subscriptions. Once the page opens click the link at the top that says Payment Cards.