If you a like hands-on approach, you can use a millimeter ruler to measure your PD.
With the ruler in hand, stand in front of a mirror and close one eye. Align the ruler's zero mark with the center of your open eye's pupil. Then, using the ruler, measure the distance from the center of your open eye's pupil to the center of your closed eye's pupil. Repeat this process a few times to ensure accuracy and record the measurements.
Incorrect alignment or inconsistent measurements are common mistakes when using a ruler.