Cool — it's really easy. After you enter your pin and hit submit below, you'll be taken to a blank version of the workbook (as if you're starting it for the first time). To pick back up with the one you saved, just click "Resume" at the top of the page and login with the username and password that you chose when you saved your form. If you forgot your password, no worries — it can walk you through retrieving it.
It'll look this ...
Just remember that each Part of the workbook is considered separate ... so when you finish and submit Part 1, you never have to log back in to Part 1 again. When you save and resume Part 2, it'll just take you right back to the beginning of Part 2, and then you can jump forward to where you left off there.