You buy some clear flexi hose (the right size to fit over the top nozzle part of a baby shampoo bottle).
Then cut off the base of the baby shampoo bottle.
Stick the hose on the top of the bottle – and there you go !
However…….. personally I would not use anything that contained chemicals and soaps (even though baby shampoo is supposedly soap free) such as shampoo etc. no matter how well rinsed.