I have been asked by a friend at work about doing her eye brows. So I put together this bit of information! ^_^!
There are 3 parts to your brows.
When your lining your brow and it goes beyond what natural hair you have you still want to make it seem as natural as possible. You want to line everything up, you can use any straight edge, like a brush, as a guide. The furthest part from your nose should be in line with your nose as in Line F on the diagram. So where the dotted line is, is how you would line up with your brush. Place a dot where it lines up.
The middle, arch, should be on a same diagonal but it should line up throug the middle of your eye. There isn't a line on this diagram.
And the beginning of your brow should be lined up vertically from the outside of your nose straight up, like Line D.
Make Up Geek has a ton of information
Some more information like the different types of brows:
Here are a few different YouTube tutorials that I've come across from different artist and how they do their brows: