Our method involves using a scale ruler to interpolate between the gridlines on a USGS Quad.  The gridlines you see on a USGS Quad shown schematically below) represent a map projection known as Universal Transverse Mercator (UTM), and are 1000 meters apart.  The small ticks on the ruler below represent 100 meter divisions.  As in any 2-dimensional representation of a 3-D object, there is some distortion;  as you view quads from various parts of the state, you will see a slight difference in the spacing between gridlines.

To determine the location of your site, print this page and fold the paper along the dotted line, and hold the ruler diagonally so that the zero and 1000 marks coincide with the gridlines.  Use the ruler to determine how many meters there are between the closest gridline to your site.  Pay close attention to the direction the gridlines increase (watch the numbers adjacent to the gridlines along the edge of your quad), then add or subtract the estimated distance to or from the nearest gridline.

The diagram illustrates what you see on a USGS Quad.  Note that a more precise measure can be obtained by following these steps with an engineering ruler and interpolating on the diagonal in a similar manner.