a). Its fairly self explanitory, the brow should begin with a vertical line that matches the start off the eye
b.) the highest point of the arch should line up diagonaly with the center of the eye and the tip of the nostral
c.) the end of the brow should should, like the top of the arch match diagonaly with the nostral, and the end of the eye
here is a befor and after example:
see she is still beautiful, now she looks more clean and careful. this is not the best example, the arch here dose not quite match the line of her browbone.
these are all general rules of what looks good, a more smooth or a sharper arch , as well as thickness, length, and a "feathered tip" may be peferable fore diffarent people.
a well shaped eyebrow should look intentional, and natural at the same time. good eyebrows say: "I am clean, I know how to take care of myself, and I care enough to do so"