You just have to bring your luggage down to the RAC desk at your resort at least 3 hours before your flight and before they close (I think they close at 1pm). They check you in just as they would if you were at the airport. An adult will have to show a photo ID in order to check in. They can print boarding passes for you at that desk. In fact, they can print boarding passes even if you are not checking in your luggage.
This is available to all guests staying at that resort. You don't need to use the Magical Express in order to use the RAC service.