A: Click the "Reorder Categories" button at the top right of the Menu Review page. We've put a red box around it below:

This will cause the category panel to open up on the left hand side of your menu.

Create a category like you normally would. Then hover your mouse over the category and press the down arrow here:

This will allow you to create a subcategory.

Same steps as above! Now when you press the down arrow, instead of typing in the title of the new sub-category, just drag the pre-existing category you already created to sit under the root category!