Mix together 1 tsp. dishwashing detergent and 1 tsp. Borax in 1 quart warm water. Put the mixture in a spray bottle.
Generously saturate the cushions on both sides with the solution. Do not be frugal -- it will clean off the mold and mildew. Be sure to get all the crevices and creases. The more you put on now, the less you will have to do when you wash it off.
Let the cleaning solution sit for 15 minutes. Then with the garden hose on high, spray the cushions thoroughly to knock the dirt and cleaning solution off the cushions. Stand the cushions up on edge to let them dry to just damp.
Spray the cushions with a fabric protector. This will both protect them and make them easier to clean in the future.
Most of the time, canvas can be machine washed and then put back on the furniture while it is damp to stretch the material back into shape. However, if you can't remove the canvas, use a scrub brush and rub it across the bar of Fels-Naptha soap and then rub the brush back and forth on the canvas. You may have to do this a few times — don't be afraid to really lather it up. Fels-Naptha soap works well for canvas awnings. It does a good job removing bird droppings and many stains.
Once you get the canvas nice and clean, take the hose or a spray bottle and rinse it off.