I'm assuming you are talking about just plain old window screens. I take mine out of the windows and lay them in the grass or on the driveway. Then fill a bucket with some water and dish soap and use a broom dipped in the bucket to wash the screens, both sides. Then use the hose to rinse. Stand them up, let them dry and replace in windows.

There are also products available for the job. Check any hardware or DIY place. You attach the plastic bottle to a garden hose and the bottle dispenses a cleaning product into the stream of water. You can clean the screens without removing them (recommend you do it twice a year - once in the spring and once in the fall).

One can go with the pressure washing and steam washing. This may be helpful.