>>519381Turn the dial left or right the appropriate amount of degrees to make up for the magnetic declination. For a -13 degree declination, I'll turn the dial anti-clockwise to add 13 degrees. Now, if I were to line up the red magnetic arrow of the compass (the one that actually spins) with the little red arrow inside the little circular housing, the BIG arrow across the body of the compass will point to actual north. (you would line up the arrows by rotating the entire compass or more likely your entire body).
Now, most good maps will provide you with the magnetic declination of the mapped region, and some will actually provide you with secondary lines indicating the direction of magnetic north, but make sure the map is recent, because the declination values are always changing.
The National Geophysical Data Center is always keeping track of the position of the magnetic north pole, so you can go to their site and find out the declination value for any spot in the world. This would be very useful if you're going somewhere where you won't have good maps.
http://www.ngdc.noaa.gov/geomag-web/#declination 
