How do you answer 'What have you done to prepare yourself to be a supervisor' in a job interview?

Presumably this is promotion, in the same company or in an equivalent position at a different company. You need to show that you know the job. Show that you understand what a supervisor needs to do. What makes a good supervisor in your company or industry? In any industry, a supervisor should be able to manage people. This means more than being liked. It means being respected. It means being able to communicate the priorities of the company to the staff. It means understanding your employees and their needs, and knowing how their needs fit in with the needs of the organization.