In the Configuration>>My Store section of the Admin App there are two configuration variables that control "One Page Checkout". When the Enable variable is set to true , the application automatically uses One Page Checkout. When set to false, the original shipping, payment and checkout screens are used.

There is another variable for enabling checkout without registration. If One Page Checkout is enabled and this variable is also set to true , then a customer doesn't have to complete a separate registration process before checking out. He can just enter the shipping address during the checkout process.