I found this bridesmaid's dress at a thrift store. It was beautiful, and fit me perfectly! But there's not a lot of weddings in my near future, so I decided to take it ($7! a steal!) and do a quick fix on the hemline.
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
Lay your dress flat, folded at the front center and back center. The side seams should rest on top of each other. Get the inside layer out of the way; we'll be working with just the outside layer to start with.
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.
Step 2: Measure & cut inside layer
Lay it back down and cut the inside layer to match. You can see that my inside skirt was thinner than the outside one, so I cut the lining to match the shorter part of the skirt.
Step 3: Hem
Double roll hems and sew.
Make the outside layer's hem very thin.
Make the inside layer's hem noticeably wider, so that it will be shorter than the outside layer.
Step 4: Hooray!
Look at that nice new cocktail dress!