Once you've customized a template, or created a design from scratch, you can save it using the 'Save' option in the editor. You will find this at the blue top bar, between the 'Resize' and 'Share' button. 

The Save button has been highlighted below.

Once a design has been saved, it can be accessed from your 'My Stuff' folder, from where you can edit it again, download it, purchase high resolution downloads, get prints or create duplicates.