Use a millimeter ruler and a mirror
Stand about 8 inches from the mirror
Hold the ruler against your brow
Close your right eye and align the 0 mm mark with the center of your left pupil
Close your left eye and open your right eye
Read the measurement at the center of your right pupil
Repeat a few times and use the average
Ask someone else to measure from the center of one pupil to the center of the other
Use a pupillary distance app or online PD tool
Get your PD from an optometrist or eyewear prescription if available