At its best:
Take cuttings from the top growth of your mint plant, about 8cm in length. Remove the lower leaves and cut the stem just below the leaf node (the point on the stem where leaves emerge).
Place the stems in a glass of water. Leave in a light, airy place until the stems have taken root.
The stems will take root within a couple of weeks. Once a good root system has developed, pot the stems up in a container with good quality, peat-free multipurpose compost.
Firm compost around the stems and water well. Top up with compost if necessary.
Trim the top growth from the stems as this reduces the surface area from which the plants can lose water. Keep the pot indoors for a week, ensuring the compost stays moist. Once the plants have become established, transplant them into their final growing positions.