Answer: I would try both methods. Sometimes they will root in water and sometimes they will root in potting soil. Cut the branches cleanly and those that you want to root in soil can be dipped in rooting hormone (found at garden centers). Those that you want to try rooting in water can be set in fresh water after you've removed the lower leaves (anything that would be submerged in water will rot). Change the water weekly.
Best wishes with your lilac.
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