Read the fabric label to see if you can hand-wash the acetate or if it is dry-clean only.
Fill a sink with lukewarm water and 1 teaspoon of mild detergent.
Gently agitate the water with the garment in it, being careful not to twist, wring, or rub the acetate fabric against itself.
Use a cool iron on the wet fabric to push some of the water out of it. Look closely to see what type of knit the garment has. If it's a circular knit, do not press an iron on it.
Lay the garment flat until it dries completely.