2. Cut off a few nice healthy stalks.
3. Plop them in a jar of water. Put the water in a sunny window.
4. Wait. Sometimes it just takes a few days, sometimes more like a week, but eventually you'll see little hairy roots start to form. Once the roots are looking healthy, find your new mint plant a home in the dirt.
The first time I did this, I fiddled around with rooting hormone, but it's hardly necessary. Mint is invasive (so plant it in pots, or else somewhere you want it to spread like mad), which is another way of saying that it can take care of itself well--and that includes reproduction.
These two little plants were leftovers from a sack of mint I bought at the grocery store a few weeks ago, but I'm planning to invade my friend's backyard for some more cuttings very soon. There's a spot in my yard where nothing else will grow (shade + Moe), and I'm hoping that this hardy little plant will be up to the challenge.