Firstly, navigate to your list of Categories.
Hover over 'Categories' you'll notice a '+' icon appear, click the '+' icon to the right of 'Categories'; alternatively, you can create Sub-Categories by clicking the "+" to the
right of any existing Category.
This will open this Category's Standard Fields.
Here, you can fill in key information about this Category (these can be edited later).
Make sure to fill the Name and Item Field, and ensure you toggle 'Detail View Enabled' if you wish to apply a Detail View, then click save.
Your new Category will show in the list on the left-hand side.