I soak my outdoor cushions in the bathtub in Oxy Clean. I use the powdered kind. Fill the bathtub with warm water and put in the powdered Oxy Clean. Mix it up really well. Then put the cushions in. The bad part is you have to let them soak at least over night. And you have to do them one at a time. They can't be all squished together.

Fabric Cushions: To clean fabric cushions, dilute a 1⁄2 cup of
bleach into 1 gallon of water. Use a spray bottle to apply the mixture,
then clean the mildew off of the cushion with a scrub brush. Once you
have removed the mildew, rinse the cushion with water. It's always food to spot test first. :)