Scrub the seat cushions with a mixture of warm water and dish soap. Use at least a few drops of dish soap---more if you prefer---in a bucket of water. Put the mixture in the bucket and dip your scrub brush in the mixture. Scrub the cushions to remove embedded dirt and mold.
Rinse the cushions. Using a bucket of clear water and a scrub brush that you have rinsed out, scrub the cushions again, this time with the clean water. As you rinse and scrub, wipe up the excess water with a clean, dry rag.
Let the cushions air dry in the sun. This will make them look brighter and smell like the fresh outdoors. It may take up to a day for them to dry if you had to really saturate them to get them clean.
Freshen up the cushions, if they still have a bad odor, by sprinkling them with powdered carpet freshener and then vacuuming. This will make your cushions smell fresh and help remove any remaining dirt from the cushions.