When the blooms feel papery in August-October, they have started to loose water and are best used for drying. See the link below for useful information on that subject.

Here is a link that might be useful: Drying Hydrangeas

They won't be completely dry until first freeze or few.

I cut my Angel Blush and Annabelle blooms in September - October, and put them in a vase full of water. When the water is gone, they are dried. If you wait too long, the florets will start to turn brown while still on the bush.

While new hydrangea blossoms don't dry well for me, I've dried them successfully once they start changing color. I've been picking and drying Vanilla Strawberry blossoms the last four weeks, as they've turned deep pink.

I hang them upside down in the house using yarn or string suspended from a hanger or one of those over-the-door hanger holders.