I soak mine in the bathtub with warm soapy water--dish soap works well. Scrub gently with a brush. Rinse well. Hang to dry. Either in the tub or laundry room. If it's warm outside you can hang them over your deck railing.
Stains or just dust? I can generally get mine clean with a feather duster type cleaner or a lint roller
Try Blue Coral Upholstery Cleaner. https://www.amazon.com/Blue-Coral-DC22-Upholstery-Eliminator/dp/B0009IR0PQ/ref=sr_1_5?ie=UTF8&qid=1545333706&sr=8-5&keywords=dry+fabric+cleaner+upholstery