In order to dry mint, I like to remove the leaves and then dry them on a clean towel overnight or until it's dry. I then pack them in a ziploc and it's ready for use.
Another trick to preserving mint is to grind it with some water and then freeze the paste. It's intense and can be used as a flavor enhancer in many dishes.
Growing up in India mint was widely used for it's medicinal qualities. To make mint oil, you follow this process.
Pluck the leaves out from the mint and wash them and let it dry on a towel. Use a mallet to tap the leaves and then put them in a glass bottle (should have a broad mouth). Pour in almond oil and let it sit for an entire day over 24 hours and then strain and discard the mint. If you want it super strong, take more mint leaves, tap them again with a mallet, put them in the same jar and pour the same oil you strained. Keep repeating this 3 times for 3 days. It will be super strong.
You can store it in the refrigerator and it should last for a long time.
Hope that helps.