First you need to obtain the Rewards Card to load on your DIsney Dollars from Visa. If you log into your account online and click through rewards stuff, it explains how. But if you are going soon (in next few weeks) I would do it at Disney (because if you do it online they mail it snail mail).
To do at WDW: Go to Guest Services (I have done this exact transaction at DTD and Epcot Guest Services, which are both indoors and air conditioned!, but any full service guest services can help you).
Tell the CM you want to get a rewards card, they will ask for your Visa and photo id. Then they type some things into computer, and hand you over the card. They also give you an envelope with instruction to call an 800 number to Visa and ask to move Disney Dollars over to it. Works great and takes very little time.
Then you use the Rewards Card just like a gift card anyplace Disney owns, and most places it operates. There's a list of where it is accepted on the Visa site.