The easiest way is to take a cab.
It'll cost about $15 plus tip.
If you do as the other poster suggested you'll have to use a park day to get into Epcot.
The other options are to take the bus from YC to the MK and then the monorail or launch to the GF, or take the bus to DTD and then transfer for the MK bus. Neither of these will require a park day use on your pass.
Personally I'd cancel dinner at the YSH and book Narcooses right at the GF instead. Better food, better service, better view.
Anne