Hi elissa,
one way is to create an item with no price in the none department. check boxes for modifier and prompt description. uncheck box print on receipt (unless you want the note on customers tab). set it to print to kitchen. Now create a quick button under touch screen configuration with the function fixed item. label it kitchen notes. enter the item name that you created earlier and save.
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Feel free to email me with more details if need be or if you want my help.