The raised ring on the compass is called the bezel, and it rotates independently of the baseplate. Leave the baseplate lined up with your route and turn the bezel until "N" on the ring points toward north on the map.

The center of the ring is conveniently transparent so you can see the map through it. Adjust the bezel until the painted lines in it are parallel with the north-south lines on map.

This step effectively records an angle on your compass — the angle between north (0°) and your intended direction of travel.