Before entering a credit card number into a customer's file, you will need to have either Stripe or Square set up. If you don't, click here for a walk-through. If you already have a credit card processor set up, go to the next step.

Go to your contacts.

Choose the customer you want to add a card to.  Click to access their file.

On the customer profile, click Actions, and select Manage Credit Cards.

On the pop-up window, click the Add or Charge Credit Card button.

And finally, you will see the last screen that will allow you to add a credit card number.