Pick the mint stalks (I cut just below the last leaf) and place in a colander.
Give the colander a good shake to remove any beasties and bugs, and then give the stalks a wash under cold running water.
Gently dry the leaves using a tea towel and remove any discoloured leaves.
Spread the stalks out on a baking tray and bake in the oven for around 1.5 hours - keeping an eye on them to ensure the leaves don't burn. You can tell the leaves are fully dried out when they become very crisp and brittle.
When they are sufficiently dry, remove the dish from the oven and then gently remove the leaves from the stalks, placing the leaves in a clean, dry airtight jar. You can then compost the stalks.