The best way to get between parks is the Hogswarts Express. In Universal Studios you must enter King's Cross station to board
As you wait your turn to board you get to go past several props seen in the Harry Potter movies
Hedwig is also waiting for the Express
At King's Cross, you actually board on Platform 9 3/4
There are signs all over to let you know what platform you are on
The train has arrived it is time to board
The train actually backs into the station
Currently they are only loading one party per compartment, which makes it a lot nicer than in the past
In Islands of Adventure, you load/disembark at Hogsmeade Station. It has a completely different feel compared to King's Cross