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.
you have to such on the one end (as with most though not all) gravel cleaners to get the water flow going - take care not to get a mouth full of fishy water ) and away you go. Works exatly like most other gravel cleaners.