Shake out your cushions, or beat them with your hand to get out some dirt and pollen. You can also vacuum them. Then, make your cleaning solution in a spray bottle. Mix 1 tsp. of dish detergent, 1 tsp. of Borax and 1 qt. of warm water. Shake well. If desired, instead you can purchase outdoor patio cleaning solution from your local home improvement store. You will also need outdoor fabric protector, which you can't make at home.
Liberally spray your cushions with the cleaning solution. You should soak them. Be sure to get along the sides and along the folded seam. Then let the solution sit for about 15 minutes. Afterwards, stand them on their edge and hose them off with a strong spray. Get both sides and the edges. This will remove the dirt, mildew and cleaner. Then, let the cushions air dry until they are just a bit damp, at which time you should spray the outdoor fabric protector on the cushions. This will help keep your cushions clean for a longer period of time and will make future cleanings easier.