The advanced tab of the configuration screen has an expancing checkout area where the checkout flow can be set. This page described the sometimes confusing legacy single screen flow. 

When a visitor proceeds to legacy checkout they see the following:

• Optional Cart Header: This can be a formatted text string entered in the advanced tab, or it can be a container from elsewhere in the site.
• A registration area.
• Editable Order Summary: This is a list of what was ordered with links to edit the items and a total including promotions.
• User Information Collection: This is a ship-to form or a visitor registration area.
• Send Order Button: This will include a PayPal message if PayPal is enabled

There is little guidance to direct the user through these areas.

The legacy flow is best used for very simple catalogs, such as ones that do not register, collect ship-to information, or payments.

If the cart is empty or if the visitor is not allowed to order the checkout screen will not show and will return to the sites current page. The site will automatically not have Proceed to Checkout links under these conditions but in obscure cases a visitor might go "back" into a cart that no longer applies.