I wear glasses for 95% of time, I want to not hate how they feel.
1) Stand in front of a vertical mirror, making sure you are square to the mirror.
2) Place a horizontal ruler on the surface of the mirror.
3) Close one eye and move your head until the reflection of the centre of your pupil lines up with zero on the ruler.
4) Keeping your head still, open your eye and close the other eye.
5) The Pupillary Distance is the reading on the ruler that lines up with the centre of the reflection of your pupil.
6) If you want to double check it, just swap the opening of your eyes and check that the other eye still sees itself at zero.
I use this method whenever I order glasses online and have yet to have a pair of glasses that have any error in the manufacturing. I now only buy glasses online.