First, please see and fully understand Ingredients/Fillings : What are they, and How-To Assign to Shapes.

To create your own ingredient, select the nozzle you would like to use for your ingredient, and then choose an ingredient that is similar in texture. In the video example, let's say you created a cauliflower mash (made with purple cauliflower!) that is similar in texture to mashed potato, but you want to fine-tune the settings for your mash. And you want to print with a 40 nozzle.

Note that this will not delete or change the original mashed potato 40 nozzle setting; that will always be in the default library of ingredients. What you are adjusting is a copy. To fully customize how your ingredient prints, you can adjust Advanced User Settings.