Do I have to finish my design all at the same time?

No. When using the Atomic Online Designer, a cookie will be installed in your browser that will automagically save your design for 30 days. If you need to work on it at different times, please go back to order confirmation email that was sent to you when you placed your order. By clicking that link again, it will populate your in-progress design. 

Please note, that this will only work on one computer and one browser. So for instance if you start designing on your MacBook Air in Safari, trying to open it up later using Chrome, will not load your progress.