If you are near-sighted, you can do this without help.  

Stand facing a mirror with a centimeter/millimeter ruler held horizontally across your nose, just below the level of your pupils.  With your left eye open and your right eye closed, look at the pupil of your left eye in the mirror.  While doing this, put the zero point of the ruler even with the center of your left pupil. Without moving the ruler, close your left eye and open your right eye.  What point on the ruler is immediately below the center of your right pupil when you are looking at your right pupil in the mirror? This is your pupillary distance, also called PD.

If you are far-sighted, and can't read the ruler markings in the mirror at close range, have someone sit close in front of you.  Use the above directions.  When your left eye is open, the other person's right eye needs to be open. When you shut your left eye and open your right eye, they shut their right eye and open their left eye to take the reading.  When setting the zero mark and when reading the pupillary distance, your helper's eye needs to be directly in front of your eye.

How to Measure the Distance Between Your Pupils