Locating the epicenter of an earthquake is a relatively simple matter. The
tools required to do this by hand are: three seismograms of the earthquake,
a map of the world, a ruler, a pencil, and a drawing compass.
On seismograms, ticks mark one minute intervals. Measure the distance
between the arrival of the first P wave and the S wave. Subtract the
arrival time of the P wave from the S wave. This number will be used to
find the distance from the seismograph to the epicenter on a special chart.
You also need to measure the amplitude of the strongest wave. Do this by
measuring from the baseline to the top of the highest line. Record this
measurement in millimeters.
Now you need to consult a chart for the finding the Richter scale
magnitude. It looks something like this 
_ _
500 100
50  
 6 50
  
 5 
  5
 4 
  
100 3 .5
  
50 2 
  .1
 1 Amplitude
202  (millimeters)
5 0
 Magnitude


0
dist. SP
(km) (seconds)
On the chart, mark the leftmost graph at the point corresponding to the
measured SP time. This corresponds to a distance  the distance from the
epicenter to that seismograph. On the rightmost graph, mark the measured
amplitude. Place a ruler or straightedge on the two marked points, and
mark where it crosses the middle line. This tells you the magnitude of the
earthquake. Do this for all three seismograms.
Next, to locate the epicenter, check the scale on your map. Figure out how
long the distance to the epicenter is on your map. Use your compass to draw
a circle with a radius equal to that distance. Center the circle on the
location of the seismograph. Do this for all three stations. All of the
circles should overlap. The point were all the cirles intersect is the
approximate epicenter of the earthquake.
Remember, the epicenter is merely the surface location of the
earthquake's focus. This method does not tell you how deep
the earthquake was. To determine that requires more information.
Of course, earthquake location is most often done on computer, not on
paper. To see an example of this, try the Virtual Earthquake program
located at http://vcourseware5.calstatela.edu/VirtualEarthquake/VQuakeIntro.html