You'll likely find a tag on the seat cushion cover or just under the cushion on the furniture itself. The tag will list care directions and may have codes on it instead of specific directions. If the furniture doesn't have a tag or any other markings, but you know the brand and model name, you may be able to track down the cleaning instructions online or by calling the company.
You'll likely see either a "W" or a "S" printed on the cushion tag or furniture. Other codes include "S-W" or "W-S" or "X," but these are more rare, according to Apartment Therapy contributor Sarah Rae Trover. The "W" code means that you can use water to clean the cushion cover. Even if there is a "W" on the cushion, you should only machine wash it if is designed to be removed from the cushion itself. "S" means you can only use a solvent on the cushion cover and should take it to the dry cleaners or an upholstery cleaner for stain removal and general cleaning. An "SW" or "WS" code means you can use either water or solvent, while an "X" means you cannot clean it at all and should only brush or vacuum it.
If the label on your cushion cover specifically states that the cover is machine washable, toss it in the laundry to clean. Household cleaning expert Heloise recommends soaking the cover in cool water with laundry detergent for 10 minutes before beginning the wash cycle. You may want to air dry the cover to prevent any shrinkage. Put back onto the cushion while still damp so that it will stretch to fit.
Keep dirt from rubbing into cushion covers by brushing or vacuuming them weekly. If you spill anything on the cushion covers, clean it up as quickly as possible, so that the stain doesn't sink in and set. You may want to turn the cushions over on a monthly basis to ensure even wear. Frequent turning extends the life of a cushion cover, according to the University of Illinois Extension.