Go to the menu in the Control Panel.
Click on the item you want add a modifier to
Click on “Modifier” and select add to <Omelette> in category <Breakfast>
Give the modifier a name and click “Save”
Click on the modifier and then click on the button with the modifier name you just created and click “Edit”
On the right hand side click on “Add new option”
Enter in the new options/Modifiers (and prices if required) and click “Save”
Your menu item now has modifiers.