Outdoor furniture covers can help protect your patio furniture and extend its life. Keeping these covers clean and well maintained can, in turn, extend the life of the covers.
The best time to clean the covers is while they are on the furniture.
Use a solution of 1 cup bleach and ½ cup mild dish detergent (Joy, Ajax, etc.) mixed into 1 gallon of water. Use a spray bottle (rinse out an old bottle from your window washing solution or something similar, or buy a new, empty spray bottle) and spray the stained areas thoroughly.
Wait for several minutes, then rinse with clean water. Allow them to dry thoroughly.
Also, use this solution in the case of mildew.
For general cleaning, use a solution of mild detergent and lukewarm water. Use a soft scrub brush or an old piece of towel or similar soft cloth to apply the cleaning solution to the outdoor patio furniture covers. Scrub, particularly seams as necessary, then rinse thoroughly, probably with a garden hose.
Dry and Fold
Be sure to allow the outdoor patio furniture covers to dry completely before folding. If the covers are not completely dry, they will mold and deteriorate while in storage, causing a more severe cleaning problem.
Source: do it yourself