With the lid removed you should now be able to easily access the inside of the condensing unit. Start cleaning by removing any debris (leaves, "helicopters", etc...) in or out of the unit that can be removed by hand. This will keep water from being trapped under leaves and such that lead to the eventual rust and breakdown of the unit's structure. Now, place a towel over the service panel area. This will not prevent all water from entering the panel but can help to catch a fair amount.
Now spray away. That's right, spray your hose from the inside out in the unit. Remember, air is being pulled into the unit thus spraying from the outside in only forces the dirt deeper into the fins. There is no need to worry about the water. These units are made to be outside so nothing your doing here is anything nature hasn't done already. Just be sure of a couple things...
Don't have the pressure set so high that it bends the fins around the unit.Do your best not to spray directly into the service panel area or at the service disconnect. Again, I'm not too worried about these items as they are rained on regularly but there's no reason to test their limits.
Once your done spraying out the dirt, you should be able to look through the fins or brushes and see daylight. That's it. Remount the lid to the air conditioner's condensing unit as it was and bolt it down.