Once you purchase you PP+, you take your voucher to the designated photo shop in either DL or DCA (which ever park you go to first). They will give you a special photo pass card (take a photo of the number on the back or write the number down and keep it someplace safe just in case you lose the card). Then you go about your day. When you see a photographer, marked on the map and at most character meet and greets, have them swipe your card and take your photos. All ride photos have to be added to the card. To do this, find your photo, make note of the number and then take it and your card to the register. They will add it. Character dining is the same as with meet and greets. When you get home, you follow the directions on the other voucher - log on to the site, make an account, put in you pp+ number and then you can edit your photos and then you can download them or have the cd sent to you.
There's a good super thread on the DL section (can't link it from my phone) but it talks about photo locations and special photos.
Hope that helps.