Blot up any excess nail polish with a clean cotton ball.
Fill an eyedropper with a small amount of acetone-based nail polish remover. Place a few droplets of the remover over the stain, but don't saturate the area around the nail polish spot.
Blot up the nail polish remover immediately, using a clean cotton ball. Use an up-and-down motion and don't scrub the nail polish, which will only embed it further into the couch fibers.
Repeat the process until the nail polish is no longer visible.
Use a dry cleaning solution and a white washcloth to blot the spot. Press the washcloth with the dry cleaning solution into the stain, then lift it up. Fold the washcloth inward so a clean portion of the cloth is exposed. Press the clean portion of the washcloth into the stain and lift up again. Continue the up-and-down process until the acetone is removed from the couch.
Allow the spot to air dry completely before using the couch.