You drink it and turn invisible for three to ten minutes, depending on the strength.

[He's not being sarcastic, he's just blunt sometimes.]

To make it, usually I like to use creep cluster vines, nirnroot or ice wraith teeth. You can also use vampire ashes, but that requires going out and killing a vampire, something I'm not really keen on doing. Or I suppose you could try and find a vampire who's already dead.