Because the lines of latitude are parallel and evenly spaced, a degree of latitude represents a constant distance on the ground. This makes plotting latitude quite straight forward.

Place the ruler so that it spans the lines of latitude the point to be measured or plotted falls between. Orient the ruler north to south. The zero minute end of the ruler should be on the southern line of latitude, when you're in the northern hemisphere. To measure the latitude of a point on the map, read the value from the ruler at the point, and add it to the latitude of the line at the zero end of the ruler. On the picture to the left, the ruler indicates the X is at the 4' mark, for a resulting latitude of 37° 34' N. To plot the location of given coordinates, make a small mark on the map to indicate the line of latitude the coordinates fall on.

Longitude represents an east-west position on the earth. Longitude increases as you move away from the prime meridian, or 0°, in Greenwich, England. Because the lines of longitude converge at the poles, a degree of longitude represents a varying distance on the ground, depending on the latitude.
Place the ruler so that it spans the lines of longitude the point to be measured or plotted falls between. The ruler will need to be on a diagonal to fit. To measure the longitude of a point on the map, slide the ruler vertically, keeping the ends on the lines of longitude marked on the map, until the edge of the ruler touches the point to be measured. You may need to extend the lines of longitude above or below the map to properly position the ruler. On the picture to the left, the ruler indicates the X is at the 3.5' mark, for a resulting longitude of 122° 3.5' W. To plot a longitude coordinate, make a small tic on the map to indicate the line of longitude. The point of interest is located where the plotted lines of latitude and longitude cross.