If you were to poll a panel of makeup artists the consensus would more than likely be that brows are the single most important part of framing a face. Unfortunately they can be tricky to groom. Here are the basics when shaping brows.
Take your tweezers and hold vertically in front of the face. The inner point of the brow should line up with the outside of the nostril on the same side.
Then look to the arch of your brow. The highest point of the arch should line up with the outer edge the iris of the eye.
Then hold your tweezers on a diagonal starting at the nostril. Your outer brow should start at the end of this imaginary line on the same side.
Once you establish where the outside brow should start, hold your tweezers horizontally and each end of your brows should line up, after arching down.