I noticed there isn't much information on this, here I'll explain with pictures! Yay!
Here is the proper way that it goes, first it's the knob, then the body, the o-ring (it's an 07 o-ring), the brass part, the round part goes facing the o-ring and the the flat part against the pin.
To take this apart, you need a 6mm socket wrench, it simply goes against the pin.
You just stick it in where you screw the tank in
When you screw it off, the knob is usually tight on the body, but you don't really need to take that off, unscrew the pin and it just comes right off, then when you screw it back in the knob goes back to normal, when screwing it back in don't screw it in tight because then the knob won't turn. Do it as tight as it can get without the knob getting stuck.