I have several patches of peppermint growing at home and though I occasionally use it for tea, most of it is now been delegated for use in my handmade mint soap. It's merrily prolific but downright invasive so I try to keep it growing in containers for the most part. All the better because when this herb gets going it can take over your garden like a wave washing over the shore.
Though mint is lovely to use when it's fresh - especially in tasty treats like Mojitos - it's more useful to me when it's dried. Using fresh botanicals and juices in soap is always risky since it can tend to go off, leaving the soap rancid and unpleasant. However in it's dried form, Peppermint gives a lovely natural colour and intense concentration of natural oils and scent.
Drying peppermint is easy, whether you too want to use it in your own beauty products or simply to have some on hand for culinary purposes. First you have to grow it and a lot of it. There's a good deal of moisture in mint leaves so the drying process will leave you with much less than you started with. For comparison, look at the top image in this post to see how much fresh mint was needed to get the amount of dried mint you see in the below image. I'd estimate that it reduces down to around 1/8th the original size and maybe even a little less than that.
After you've gathered a sizeable amount make sure to wash and dry it thoroughly. Then lay it over a lined baking sheet and heat it into the oven at 80°C/175°F until you're sure it's completely dry. It takes about an hour with a convection oven but can take over double that time with a conventional appliance. The mint will initially wilt and lie in sodden heaps on the tray so if you turn the mint over a few times during the drying process you can reduce the time somewhat.
Though you can remove the leaves from the stems before drying I don't really bother. The softer stems can easily be crumbled into usable pieces so definitely leave those in even if you've opted to remove some of the woodier bits. And if you're using the dried mint for tea you can even pulse those with a hand blender and then mix it in with the crumbled leaves. Store your dried herbs in dark, dry containers and they'll be good for at least a year.