Gardenia stem cuttings are easy to start. You need cuttings from this year's growth. Rather than explain in detail how to do this, I've attached a couple links to help you. Be sure that you follow the sanitation rules, just as you would for orchids. Rootone rooting hormone works quite well on cuttings. You can usually find a small package of it at most gardening centers for a couple dollars. Do not reuse any of the hormone or put back into the container. Many years ago, we started new plants from forsythia bushes and also azaleas. Same procedure. Good luck.
http://www.clemson.edu/allendale/local/pdf/stemcut.pdf http://www.briansgarden.com/2002/10/...-gardenia.html