It's important to know what kind of cloth it is first. There may be tags on them that you can still read that will tell you either what they are made of or the care instructions themselves. Canvas can be machine washed as long as it is put back on while still damp to retain its shape, and never dried in the dryer.
When in doubt, this should always be safe: (found in multiple places on the net)
1 teaspoon dishwashing detergent
1 quart warm water
1 tablespoon borax
Put it in a spray bottle and saturate cushions on both sides, paying particular attention to the crevices and creases. Let the cushion sit for 15 minutes. Spray the solution off with a hose using a strong force of water. Stand each cushions on its edge out of the direct sun to dry. When almost dry, spray with a fabric protector. To remove dust between cleanings, vacuum as needed. Or, if the cushions are small enough, just shake them out.
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