1. Download the .png file from the website. This is more than just copying it from my blog, you must download the file as a .png for it to work for you.
2. Use a program that work with a .png file. I use Photoshop, but PSE works too, as well as picasa and paint.
3. The cards are like physical layers, imagine you are really scrapbooking. Put your photos beneath the .png overlay. You can change the order of your photos and the card by dragging the side bar (as shown in the illustration below).
4. Once you have completed your cards, upload them to costco.com, (upload them at the slower speed), and then print.
The internet is a great resource. I have learned the majority of my photoshop skills from researching it on the internet. If you have questions, play on google, be patient. Ask friends, ask me, ask on internet forums, ....there are some great resources out there.
Good Luck and Merry Christmas.
I have changed this card to be blank where the year is. You can simply use the type tool to add the year to the card. I typed mine in white, an then changed the opacity a bit.