Hi Free Rider,
To make essential oils you need a distillation apparatus (a still). Another way is using a Sohxlet extractor (more efficient then a still).
Mint oil is often prepared at the plantation where they collect the leaves and place them in sealed vats and inject steam. Then the escaping steam is condensed back to water using a cooling coil. The oil that floats on top is separated. the cultivar of choice is corn mint or mentha averensis.
Using a still at home you can do the same.
To obtain pure menthol crystals, you must refrigerate (even freeze) the oil and menthol needles will appear. On an industrial level these crystals can be several inches long. Smelling menthol crystals makes your eye feel cold.
Maybe an easier process would be to make a tincture.
Here is a good explanation:
Homemade Medicine | These healthy herbal tinctures are easy to make at home.
Good luck!
Luc H.