In order to create a brow mapping, you have first define the fixed points. To do this, you take a pen and place it along the nostril up to your brow. That is where it should begin. If you put the pin from the nostril at an angle to the pupil, you get the highest point of the arch. You can find the outermost point if you think of a line from the nostril over the outer corner of the eye. If you connect the dots with a light-colored pencil and draw an ideal eyebrow, you can see exactly which hairs need to be removed.
Here you can find more inspiration about Brow Mapping: