Start by mixing 1 cup of vinegar and 2/3 cup of water in a spray bottle, then apply the solution to the affected area. Let it sit for a few minutes before scrubbing it with a brush and rinsing. If any mildew remains, you can mix 1 cup of bleach with 1 gallon of water, then soak the area and let it sit for several minutes. Scrub the stain with a stiff-bristled brush then rinse with clean water and repeat until all mildew is gone.