Take a 10ml bottle and cram it about 2/3 full with menthol crystals. Top it off with PGA (pure grain alcohol, like Everclear), cap the bottle and shake the bejeezus out of it until the crystals are all dissolved. Done.

This stuff is super strong, one drop will mentholate 10ml of juice easy, and won't thin it down too much since it takes such a small quantity to do the job. You can also mentholate just about anything with it since the alcohol evaporates quickly but leaves a menthol residue behind when dropped on something dry, so you can use it on cigarettes, pipes, incense, the cat... whatever. A great addition to any DIY kit.