Seat Height: Measure from the top of the saddle above where the seat post ends (a), to the middle of the bottom bracket (b).
Set Back: Plumb bob from a straight down.  Now, measure the distance that point is behind the bottom bracket (b).
Reach: Measure from point a to the middle of handle bar (d).
Drop: Take a broom stick and sit it level on the seat so it hangs over the handle bars.  Measure the distance between the broom stick (d) and the bars (e).