Applied online.
You select a date on which you can attend an interview session. The process is pretty basic and simple. There is a group of applicants scheduled to interview on that date, at that time. All of the applicants are shepherded into a makeshift "waiting room" (at the particular office where I interviewed, the room was a general conference room). A group of UPS HR personnel (in my case, it was either 3 or 4 HR people) interview the applicants one-by-one while the rest wait in the room.
The interview was very straightforward. The interviewer asked a couple of questions about my resume (probably because I was over-qualified for the position), then described the job responsibilities, likely average hours per week, and finally asked if I had any concerns, issues with job requirements, etc. They make copies of your two forms of ID and conduct a standard background check. Finally, assuming you're ok with everything, they schedule you for an orientation session (mine was the next morning at 7 am) and have you fill out some online forms prior to your orientation session (you must do this before your orientation session; if you're unable to do so, you need to reschedule your orientation for a later date).
The orientation session -- for which you're paid at whatever hourly rate you were hired -- consists of a short PowerPoint presentation that cover the basics of the job (e.g., where to leave packages, when signatures are necessary, etc.), and a handful of short (2-5 min.) videos that cover safety (proper way to get on/off truck, lift & lower heavy packages, etc.). Again, very basic.
At the completion of the 70-90 minute orientation session, you're available to be called as a driver helper.