I was contacted by a Professor from a previous class that I had taken about being a teaching assistant. I filled out a form for the job and got an email a couple of days later that informed me that I had been accepted for the job.
I applied through other source. I interviewed at Purdue University
Interview
Easy, there was no interview at all. The chemistry department trusts all students and gives them a teaching job if their research professor is not paying them a research assistantship.