Start by clicking the image you've chosen for your prints:
A new window will open containing an enlargement of the image you've chosen. Here, you can inspect the image a little more closely and preview different color choices. You can also rotate the image, or you can use this interface to page through other images in your photoshoot.
Select the work print size you desire using the drop down menu at the top of the window.
When you are ready to proceed, click the "add to order" button. This will take you to the next page, where you can select quantities as well as expand your order. Don't worry. You'll be able to return to your proofsheet to select further images.

You will then be taken to the ORDERING PAGE.