I applied online. The process took 1 week. I interviewed at University of Memphis (Memphis, TN) in Feb 2016
Interview
I applied online and got an email regarding scheduling interview then had an interview one on one and heard response next day with an offer. The process is not that difficult it is just depends on when the professor is available and how much time they have to get back to you.
it was technical, free flowing and smooth. You basically have to sell yourself as being inquisitive and passionate about research. Make sure the professor you are reaching out to has interests that align with yours.