Remove the sprayer top from a clean spray bottle. Fill the bottle ¾ of the way with tap water. Squirt approximately 1 drop of grapefruit seed extract oil per 2 ounces of water in the bottle, or 8 drops of oil in a 16-ounce spray bottle. Add a few more drops if you suspect the mold or mildew may be stubborn to remove. Replace the spray top on the bottle and seal tightly. Shake the bottle to mix the liquids.
Turn the spray nozzle to one of the powerful stream settings, such as jet or stream. Spray the grapefruit seed mixture directly onto the mold or mildew, using enough spray to saturate the area thoroughly. Allow the spray to soak in place; it isn't necessary to rinse.
Reexamine the mildewed areas after an hour or so. Wipe with a damp, clean sponge, if desired, though wiping is unnecessary. Spray the grapefruit seed extract mixture around all areas where mold and mildew occur, as it will help prevent the problems from recurring.