First, go to SnapMemories' website.
Sign up with your name, your Snapchat name, and your email address.
Once you verify your email, you just have to add "mysnapchatmemories0" on Snapchat. This will let SnapMemories have access to and save your stories.
That's it! On the first day of every month, the service will send you an email with a link to let you download the previous month's Snapchat Stories. Since SnapMemories is brand new, you'll get your first email from the service on March 1. You'll never miss any of your Stories again.