Step 1: What You'll Need

All you need to make the mixture is:
Water
Baking soda

For every 1 cup of water add a quarter of a teaspoon of baking soda and stir it well.

It's that simple!

Now all that's left to do is to pour it into the spray bottle and apply it to your target plant.

Step 2: Apply to the Plant

Spray the solution liberally over the desire plant (try to get the underside of the leaves also) until it is covered. Ideally you want to repeat this process twice daily or more frequently if you get around to doing it, but don't worry if you miss a day. The great part about it is that (so far as I can tell) the spray does not irritate the plant at all, so you can feel free to apply it as little or often as you want and in a week or two that plant will be free of mildew!