The simple answer is you mail postcards from the parks to people who have asked for them to be sent. Most people post an offer to be a Fairy Godmailer here. Be sure to have people PM you with their information. We don't want personal info posted on the boards. It's a good idea to set a limit on how many you're willing to do. I've seen anywhere from 3-10. You ask for the name and address of who it is going to and any specific message they want on the card. Some people ask if there are specific characters/scenes they want on the cards, but be careful there. The postcard choices at WDW are pretty limited. You can find princesses (not always individual ones, but groups) and the basic, Mickey, Minnie, Goofy and Pluto (again in a group). I always ask for a second choice and let the person requesting know I'll do my best! Some people stand in line and actually have the character sign them, but others write the message and sign it themselves.
I enjoy doing it. It's fun and a way to spread the magic!
Hope this helps.