1 quart warm water
1 tsp blue Dawn
1 Tbsp Borax
Mix in a bucket, sponge solution onto the cushions, then scrub. Soak 15 minutes and rinse with hose.
Here is one to try: https://www.hometalk.com/23225743/how-to-clean-your-patio-cushions-easily
After thoroughly cleaning and drying them, give them a good spray with Scotchguard. This will help prevent staining.