Hold instrument level, 12"-18" in front of body and at elbow height, so that operator's head is just outside of grid area. Assume four equi-spaced dots in each square of the grid and systematically count dots equivalent to quarter-square canopy openings. Multiply the total count by 1.04 to obtain percent of overhead area not occupied by canopy.
The difference between this and 100 is an estimation of overstory density in percent. Assuming each dot to represent one percent is often accurate enough. Make four readings per location - facing North, East, South and West - record and average.