Hello. This link might be helpful.
The easiest way is to remove the leaves and hang them in a cool, dry place.
Hi Diane, hope this is the information you need to preserve your hydrangeas.
* One can cut the blooms, strip off the leaves, arrange them in a vase, with or without water, and leave them to dry. It is not necessary to hang hydrangeas up side down to dry unless the stems are very thin and weak. * To retain extremely natural hydrangea color, use Silica Gel to dry fresh blooms.
plantaddicts.com › All About Hydrangeas
Tie a string around the stem and hang upside down. This is an in general how to preserve flowers in a dried state. If you want to press, you can place between layers of wax paper and then put two books, one on either side. I'm not sure how that would work on hydrangeas since they are so round.
The blooms can be put into a plastic container of silica or white sand for 2-4 days, then take them out and gently shake off. For best results cut them at the end of the season when the stems begin to dry, if there are some browned flower petals, they can be trimmed off.
If you wait until they are almost dead and then cut them and put them in a vase they will last like dried flowers