Place a mm ruler up against your eyes, resting it on the bridge of your nose.
Line up the starting point (0 mm) with your left (or right) pupil. This measurement is only accurate if you are looking straight ahead.
Look at yourself in the mirror or have someone else read the ruler. If someone else is reading the ruler they must be at the same height as you.
When the 0 mm mark on the ruler is lined up correctly on the left (or right) pupil the mark that lands in the center of the right (or left) pupil is your distance PD.
Repeat these steps 2 or 3 times to make sure your results are accurate.
It is highly recommended to get your PD from your optometrist.