With a ruler/ something long and straight (no pun intended)
Firstly: Line from the corner of your mouth to the outside of your eye: Any hairs after the object should be removed.
Secondly: Line up your ruler from the corner of your mouth, straight up, in line with your nostril: Any hairs after this ruler should be removed from inside your eyebrow
Thirdly: Line from the middle of your eye upwards where your pupil is. ; This should be where your arch should be and tweeze.
NEVER; Overpluck eyebrows (they tend not to grow back) or Shave the middle of your eyebrows.