In the Issue Category configuration, you will have the opportunity to add fields to a stage. This comes after you have built out your stage/transition process. Included under each stage you will see button. When you click on this button you will see a window that offers you 3 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 allows you to add one of the account custom fields that you created Creating & Managing Account Custom Fields. You can only add an account custom field one time to a 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.