Unless you plan on growing rosemary inside, you will need to obtain cuttings from a rosemary that is hardy for your area. If you do not already have a plant to take cuttings from, you can likely find a friend or neighbor that will be happy to let you have some cuttings from their plant.
Place a mix of seed starting mix and sand (75% / 25%) into a fairly deep container (6" deep) that has drain holes. Water thoroughly.
Cut several ends of new growth on the rosemary plant. Allow for a certain number of them to fail (although I now have almost 100% success rate) Each cutting should be 4-5" long.
Carefully strip the rosemary leaves off of the bottom 1/2 of each cutting - leaving the leaves near the top. Some people use rooting hormone although it can impede rooting with some plants. I have had better success without it for rosemary than with it.
Evenly poke holes in the soil for how many cuttings you have. You can place 12 cuttings in a 5 x 5 container. Place one cutting in each hole (stripped end down) and firm the soil around the cuttings. Water well (again) Place in an area that will not freeze where it gets some light.
Check the cuttings in 2-3 weeks to see which ones have rooted. Plant the ones with roots in their own potd and treat as a rosemary plant. Some times of year the cuttings will root quicker than others. It can take 2-3 months in the late fall / winter and as little as 3-4 days in the spring for the cuttings to root.