Shift stick to right or left side of the tree until the zero or left end of the volume side of the stick lines up with the left edge of the tree. Sighting the right side of the stick where it touches outside bark (only moving your eyes) gives you the diameter on the top line and below this the number of board feet for trees of different numbers of logs.
Say you scaled a 16 inch diameter tree with three logs. If you have a Scribner scaling stick you would calculate that the tree has approximately 226 board feet. To accurately measure lengths and diameters, you must hold the stick in the exact vertical or horizontal.
To measure volume of logs: Position the "log diameter" scale across the small end of the log by placing the stick across the place that appears to be average diameter (or take several readings and average). Log volumes for different diameters and lengths from 8 to 16 feet can be read on the flat side of the stick marked "log scale".
Say you scaled a 16 foot log that averaged 16 inches on the small end. Looking at the log scale where these numbers correspond you would read 159 board feet Scribner log rule.
Logs over 16 feet long are scaled as two logs allowing for taper on logs 22 feet or longer. A 20 foot log, for example, 15 inches in diameter, would be scaled as two 10 foot logs, each 15 inches in diameter.