Turn off the electricity to outside lights and outlets attached to the house. Cover any plants that may get cleaners dripped on them. Remove any furniture or items stored near the house.
Mix 1 qt. of bleach and 1/4 cup of a good quality dish soap to 1 gallon of water. Stir well.
Use a garden hose and wet the section of the house down where you intend to start cleaning.
Place a long-handled, soft bristled brush in the bucket of cleaner and start at the top of the siding. Scrub small sections and dip the brush back in the solution.
Rinse off the cleaner before the siding dries. As you rinse, wet the next section to clean. Work your way down so you don't leave any residue on the siding.
If you still have dark spots of mildew, use a commercial mildewcide to rewash those areas. You can purchase mildewcide at paint stores and home improvement stores.