How to measure creepage distance?
Creepage is along the surface, "as the ant walks", and clearance is shortest distance, "as the bird flys."



(Image from here, along with some good reading material)

In practice, creepage and clearance on a PCB are the same initially. The minimum clearance creates a no man's land between the two sides of the isolation barrier. Nothing conductive can be in that area. If more creepage is required, a slot can be milled in the PCB at the appropriate point. It's fairly common to see slots under isolating components that cross the isolation barrier.