I just got done doing this. I pulled the 2-piece mouth hole out with a tiny pin. There are 2 pieces; the one you can touch with your tongue and another inside that's held in by the first. Then, as said before, pop the tank cap off. I used plain, white vinegar, then rinsed with hot tap water. ....-n-span!
The vapor is emitted outside the tank and drawn into the mouthpiece through holes on either side of the tank where the flat veins are molded in the side. I had some juice that got into the mouthpiece, so I totally disassembled the tank and cleaned it thoroughly. A tiny smear of petroleum jelly (Vaseline) will seal the cap if you have leakage issues between the cap/tank.
Hope this helps answer your question. Good luck!