Part-time supervisor position is filled within the company. Part time package employees can apply at anytime if there is a position available. The best way to get an opportunity (at least with my location) is to be a hard worker or at least gets along with the current part-time supervisor. Part-time supervisor will make a recommendation to the full time manager which makes it easier to get a chance. If your initial submission is accepted then the next step is to take a test to see if you have the qualities to become a supervisor. The test is online and consists of situational questions in the work environment. The answers are in multiple choice (fill in the blank too?). A good way to take the test is to be neutral in your answers. You shouldn't be too extreme or mellow in your answers to the questions. After you pass the test, the full time supervisor will give you a face to face interview. Depending on that, you'll get some more management training and start your new position.

They only hire from within. The interview process is more about you understanding the actual role and commitment of the job. If you're picked for a part time supervisor interview, you already got the job.