Once your custom content type is defined, custom fields can then be added.
As is often the case with Drupal, there is no "right or wrong" here. How a custom content type is built is completely determined by the person building it.
The first thing we need to do to add fields to our Supplier content type is get into its field manager. To do this, from the Content type manager, in the Supplier row, click on the manage fields link.
You will then be taken into the Supplier content type's field manager...