Mix about a gallon of warm water with detergent and a quarter cup of bleach. Put on your rubber gloves and begin scrubbing.
Scrub one section of cedar siding at a time working from top to bottom.
Once you have scrubbed a section of cedar siding it is time to break out the power washer. If you do not own a power washer you can rent one at any hardware store. You will notice that most power washers allow you to ad a cleaner. For cedar siding, it's generally more effective to do your scrubbing with a soft brush first and then power wash with water.
Set up the power washer according to the manufacturers instructions. Use the power washer on the lowest possible pressure for cedar siding. The cedar wood can be damaged if you use a high pressure. It's tempting because you may think the higher the pressure, the cleaner your wood will be, but this is just not the case.
Start pressure washing with back and forth strokes working from the top of the cedar siding down to the bottom of the siding. Continue until all of the cedar siding is clean.