You can use any beer even leftovers.
Use a basic shampoo, not the "full of chemicals and other additivies".keep it simple
(thanks fotograf.nu for the pic,sry my cam is stupid)
Reduce the beer down by simmering it in a pan for 20 mins. or until reduced. Let it cool to room temperature and add it to the shampoo.
You can pour this straight into an empty shampoo bottle or a funner way pour it into a beer bottle and secure with a cork or bottle top.