You will get a photopass card from the first pp photog you visit. You then keep that and they will scan that card every time you have a picture taken (You will want to record the number on the card in case you lose it). If you happen to forget your pp card, just get another one.
You will then go onto the PP site and retrieve your pictures, you will enter in your pp card numbers, and you will then be able to view your pics, add sigs, borders etc etc. When you are done doing that, you will then 'order' your CD, it will pop up at the normal price, when you enter in your coupon code you got after pre purchasing the PP CD it will take the total to zero.