If you've completed a round you get an overview of what you've sold right? Then there are buttons next to the amount sold (the ones with the ? in them) that indicate what the complaints of the customers were.
Say you get a complaint about Malibu, then that could mean that you either have too much Malibu in your cocktail or too little. It requires some trial and error to find out what the customers like and it may be influenced by the kind of music that's being played too. Different music attracts different people.
For more information on that game you can always check the instructions posted
here
"The absence of rules is the enemy of art." ~Orson Wells
