Select the delicate cycle on the washing machine and make sure the water temperature is cold and the spin is on low.
Can you machine wash rayon. If you must machine-wash a rayon item turn it inside out and put it in a mesh washing bag. The biggest cause of rayon items shrinking is because you used warm or hot water and a low to high dryer heat. Fill a clean sink or washbasin with lukewarm water then add a few drops of detergent preferably one made for delicates.
For instance wool is notorious for shrinking. Alot depends on how the fabric was treated by the manufacturer during garment construction. Understand that rayon fabric may shrink or deteriorate in the machine especially if it is not recommended for machine washing.
Some manufacturing processes produce colorfast sturdy rayon fabrics that can be machine-washed. Dry it on a flat surface. Wash in cool or temperate water with your han-washing detergent.
However untreated or viscose rayon should not get wet so it needs to be dry. Its best not to machine dry a rayon item as it can destroy the fabric. It will need the short delicates cycle in your washing machine on a medium to low temperature.
If you must machine-wash a rayon item turn it inside out and put it in a mesh washing bag. Rayon and viscose are like mogwai in that you should avoid exposing both types of fabric to water. As with all fabrics if it was already pre-shrunk it will be fine.
Machine washing can be damaging for some heavy clothing such as wool sweaters and jackets. Do not put it in the dryer as Rayon can definitely shrink in the dryer. If you opt for machine washing you should turn the viscose item inside out and place it in a Mesh Washing Bag to avoid snags and tears.