Holly, I had never done this before but I just tried it out and found it to be very simple. I downloaded their desktop app from here: https://www.amazon.com/Amazon-Photos/b?ie=UTF8&node=13234696011
Then I logged in to my account where I had a few test images.
Then I clicked the download option and told it to download all my images to my hard drive, as you can see in this screenshot. It will probably take a long time if you have a ton of photos, but it was easy to initiate.