It depends on the degree of dirt. Spray with a cleaner like GP66 and scrub. Pressure wash for more build up.
Each spring I get make a bucket of soap water with dish soap and a good glug of vinegar in it. With a terry rag I wipe down the patio furniture, then hose it off.
To deodorize and inhibit mildew growth on outdoor plastic mesh furniture and patio umbrellas, mix 2 cups white vinegar and 2 tablespoons liquid dish soap in a bucket of hot water. Use a soft brush to work it into the grooves of the plastic as well as for scrubbing seat pads and umbrella fabric.
Pressure washer works great alternatively you can wash by hand using detergent and a scrubbing brush.