How should glasses sit on my face?

The right pair of glasses should sit comfortably on the bridge of your nose. They shouldn't press against your eyebrows or cheeks but shouldn't rest so far towards the end of your nose that they slip when you squint or wrinkle your nose or keep slipping when you look down.
Your eyes should be directly in the center of the lens, from left to right, and fill the top half of the lens. The bottom of the eye should roughly touch the vertical midpoint of the lens.