You will need to have a Disney account ( on the WDW site) then when you login go to the My Disney Experience section and can link the park tickets there, once you hit the 30 days you can pick your fast passes. Also if you have a Magicband you can link it in there too. There is also the MDE app than can be downloaded onto your phone as well so while in the park you can swap fast passes about and if a ride goes down that you have a fastpass for the app will inform you and you will be given a general fast pass to use on any ride.