You preorder at the disney photopass website.
http://www.disneyphotopass.com/previsitcdplan.aspx
There are pretty easy instructions to follow. When you get to WDW you'll see Photopass photographers all over at all the parks. Sometimes they'll approach you other times you can just go up to them. If you have older kids, make sure they know who Photopass photographers are so they can get pictures taken of them if they go off alone. You can consolidate the cards onto one main one so no worries if you end up with a few of them.
To enhance the photos it is all pretty easy and done online at the photopass site once the photos are up. Each park has it's own little extras for the photopass pictures so you can play around with them quite a bit.
You pay for one CD but if you end up with 500 pictures and then you enhance all of them and make the enhanced ones 'duplicates' you would then have 1,000 photos. Of course that is too many for one CD but they will send you however many CDs it takes to fit all your images.
If you have questions about how to enhance the photos, stop in to the photo shop at the park and they will show you. They have a few terminals onsite to view your photos but it can take quite awhile to enhance them. But, if no one is waiting for a terminal and you have time I guess you could spend hours there.
It is a pretty good deal b/c you get the CD and can print the images or put them on your photosharing site for your friends/family, along with all the rest of your pictures.
But really make the effort to find the photopass photographers and have as many pictures taken as you can. Big group, just the kids, just mom and dad, just the girls, just the guys, things like that. Some photographers are better than others so don't feel like that you can't have a picture of you down Main Street every day you enter the park. Or in front of the Tree of Life at Animal Kingdom 5 different times. Even at different times of the day because of the light.
Really, they are all over the parks so you can get great shots. At Epcot when the characters are out there is always a photopass person with them. Keep your card always at the ready so they can quickly scan it and add the photos of the family with the character(s). At Magic Kingdom, when the characters are out I found that they usually did not have a photopass person with them, but there is always opportunity to use your own camera.
Definitely a good deal if you are going to WDW for a few days.
Best Wishes, and hope I answered your questions.
and am wondering how can you preorder the photopass cd? also is it worth pre ordering? will u get lots of good photos on them? also if you pre-order how can you do the borders and things? any help would be awesome
can't wait to get to the magic 
already counting the days till we leave 
