If you click on Add Subscription button under recurring you will be brought to this page. You can fill out this information and then add this customer to your customer vault for future use.
Another way to get to this page, is in your customer vault, choose the customer you wish to sign up, and click on the button.
This will automatically fill out the information on this page with the information from the customer vault.
Once you click continue, you will choose or create a new plan for your customer.
The plan is what determines how much and how frequently the customer will be charged.
Hit save and your done, your customer has been set up for reoccurring billing.