When you have found an item that interests you, click the basket image next to it, to place it in the shopping cart.
With the cart you can continue picking up items by pushing the button "Continue Shop". When you have finished, just like going to a cashier, you verify the items in the cart. At this point you can insert, cancel or confirm what you have selected by pushing the button "Begin Order". Then the pages for personal data and for order completion will appear.