Click on Relationships on the left hand side of the screen.
Click on Manage Suppliers.
Click on "Add Supplier" on the right hand side of the screen.
Select Supplier from the drop-down box and type in the details of the customer. Then press "Submit".
If all of the fields were filled in correctly, congratulations you have successfully added a Supplier. If there is something wrong, an error message will appear and it will say if a field was filled out incorrectly and you might need to make a adjustment to it.