Rooting hydrangeas is fairly simply and, with the right steps, could be successful for you as well. There are different methods to propagating hydrangeas via cuttings, some in water and others in soil. It all comes down to preference, but rooting in water is sometimes more difficult than just replanting them.
To start, cut a branch about 5 to 6 inches long. It is up for debate, but some gardeners say that it works best if you take a branch that didn't flower that year, though this decision is more of a personal one that one founded on fact. Then, remove the leaves of the bottom two leaf nodes. The leaf nodes are where the three leaves meet together at a common point. The leaves that you allow to remain should be cut down to half their original size.
The next step is optional, but some experts suggest that you dip the cuttings in root hormone and then insert into some sterile medium, such as coarse sand. Although optional, many gardeners have expressed that this helped with their rooting tremendously. Rooting hormone will give your plant that extra push it needs to start reproducing its rooting system.
If you are going the potting route, place in pot and water and allow to drain. When you cover the pot with plastic, be sure that the soil is moist, not sopping wet. To keep the plastic from touching the leaves and burning them, use stakes to prop the plastic up. Never place the tray of cuttings in the sun. Combined with the rays of the sun and the plastic, you'll cook the cuttings to death.
You should notice some growth within a few weeks. keep an eye on them. Once they reach a decent size, 4 to 6 inches in height, you can either transplant them to another pot to allow them to grow further or you can transplant them outside.