Experts and BHG readers answer.
The best way to root hydrangea cuttings is in moist soil.
Take a fresh 5-6 inch cutting and remove the bottom leaves. Dip the cutting in a rooting hormone and place in damp (not wet) soil in a container. Cover the container with a plastic bag to keep in moisture. Put it in bright light but never in direct sun. Expect the first roots in 2-3 weeks.