In the stage of the category click on add a new field to this stage.
There are three stage field options:
Add Plain Text/Instructions - this will allow you to add just that. It will be a text box that you can add instructions on what needs to be done within the stage or any other applicable information.
Add an Account Custom Field - this will allow you to add one of the account custom fields that you created. You can only add an account custom field one time to each Issue category.
Add a Stage Custom Field - this allows you to add a specific field that will only be used in this stage in this category. This is the same process as creating an account custom field.