So here's how you turn a two-layered standard bridesmaid's gown into a cocktail dress fit for a good party.
Step 1: Lay it out, measure & cut outside layer
Decide how high you want the hemline to be. I went with 18" shorter in the front, 8" shorter in the back.
Measure straight from front to back, and cut just below that line. (Leave a seam allowance.)
If you're nervous/unsure of the hemline you want, pin along the line and try it on again.