PD or Pupillary Distance is the distance in millimeters between the center of one pupil to the center of the other.
Stand in front of the mirror around 8 inches away and hold the millimeter ruler on top of your eyebrows.
Look straight then close your right eye. Align the "0" of your ruler to your left pupil's center.
Use your right eye by closing your left eye. Take the measurement (in mm) that you see above the pupil of your right eye.
Note: For accuracy of your PD measurement, it is recommended to repeat these steps at least 3 times.