instead of writing some abstract explanation here – there is also a “how to” tab in the add ingredients plugin itself btw – have you looked the the (user)examples of the demo and looked at the screenshots ?
https://demo.wp-pizza.com/wppizza-add-ingredients/our-menu/user/

essentially and in a nutshell:
– you add the ingredients you need (i.e all the frostings and all the chocolate chips etc)
– depending how your meal sizes are organised (one for cupcakes and one for cookies for example) assign the relevant ingredients to those “meal sizes / groups” and enable add ingredients for the relevant item.
for more granular control you can also/additionally use custom groups

there are different ways to organise things – consider “meal sizes” as the parent of menu items and ingredients which essentially links them together so to speak – it’s kind of up to you to decide what works best for you though