You can apply it any time that the ground is not frozen. Water before and after applying it. Bottled water may be ok but it could also be expen$ive. Consider getting a rain barrel instead (if you can make it fit into your budget). I use garden Sulphur instead. It should be applied every 6 weeks or so but I only apply it yearly (my aim is not to change the color but rather to prevent chlorosis).

I've never tried this for hydrangeas (I like mine pink!) but I do this for my blueberries since they need acidity and my water is alkaline. Fill a watering can with 1 gallon of water, then remove a pint of it. Add a pint of vinegar, and water your plant. Works for blueberries to counter alkaline water, so i think its worth a shot for your hydrangeas!

Oh, I just ate a "blueberry shortcake" a few days ago. Thanks for making me remember that! It was very good!