Solve the maze that starts in Thunder Mountain to get to the left side of the river.
Ride the monorail to the Frontierland stop to get to the right side of the river. The easiest station to get on the monorail from is the Adventureland stop. You get to the Adventureland monorail station by going inside the big skull in the Forbidden Temple.
qruthie (the 'q' is silent)