Oh, how I love them! They are exquisite show-stoppers even if they can be tempermental at times. I've been growing hydrangeas for the past twelve years and I'm still trying to learn how to get better results in my garden.
If you are lucky enough to have some in your garden and are interested in drying some, now is a good time to go out and give them a touch. If they are starting to feel papery, then they are ready to cut. If they still feel soft, then it is too early to cut them - give them some time.They should be ready by the end of August.
If your evenings cool down (that just doesn't happen down here), you can cut them at that time. If not, then wait until the morning. You want to avoid cutting them in the heat of the day. Use sharp pruners and cut the stem diagonally. Try to get a long stem, but if you see any nodes (I call them nubs) appearing where the leaves connect to the stem, be sure to cut at least an inch above these nubs. These nubs could be next year's blooms. Most mophead hydrangeas bloom on "old" wood and I never cut mine after September for this reason.
As soon as you've cut your bouquet, take them inside! Do not go get the mail, do not stay out and water your other plants, do not pass GO! Sink them in water and just leave them. That's it - other than checking to be sure the water level doesn't go below the end of the stems.
This is a Penny Mac hydrangea from my neighbor, Pam's beautiful garden.
Same shot, but edited to add some color pop.
This one I went with the vintage look.