Look through the glass with the black circle. Use the levels inside the tool to make the long section perfectly upright (level in both planes). While holding the tool still, look into the tool at the cross hatches (there are mirrors inside it so you can see around the corner) and determine whether or not the intersection of the black lines has sky or vegetation behind it. If there is sky, it is a negative for canopy cover. If there is vegetation, it is a positive for canopy cover.
A downloadable PDF of How to Use a Densitometer is available by clicking here