1. Go to Settings > Billing.
2. Click the Default card on file card image (or Manage cards / Add a card under it). The Cards on File window opens.
3. Click Add New Card.
4. Enter the Card number, Expires date and Code (CVC) in the secure Stripe fields and click Add Card.
5. Confirm "Add new card - Are you sure you want to add this card to your account?" with Add Card.
6. You see "Success! Card added with success." and the card appears in the list. If it is your first card it becomes the default automatically; otherwise click it to make it the default (see the next question).

Card details are sent straight to Stripe from your browser; Sprwt only stores a token. Card errors (wrong number, expired) are shown in red inside the window.