We bought two Norwalk chairs at a consignment shop four years ago. Apart from a handheld steamer, I hadn't cleaned the cushions at all. Now, a rich tapestry fabric hides a lot of dirt, but even I could see the peanut butter smears.
I had neither time nor money to rent a steam cleaner for the day. Instead, I filled up the bathtub with warm water, added Oxiclean Baby laundry detergent, and soaked the cushion covers. The water INSTANTLY turned brown, making me feel like the worst housekeeper on earth!
It took three more tubfulls to get clear water. I let the covers drip for an hour, then laid them on a sheet in the sun to dry. While you're at it, air the cushions in the sun, too.
Insert the cushions while the covers are STILL DAMP. Otherwise, the fabric may not fit back on the cushions after drying. Cotton may shrink more than synthetics.
This method did use a lot of water, but I'm very pleased with the result!