Applied online through my school and then a recruiter contacted me via phone. The recruiter offered some very high-level details about the company and told me next step would be a quick phone interview and asked whether I wanted to do it right then or at a later time. I chose right then and the recruiter ran through my resume with questions like "Walk me through your resume, tell me why you chose your college major, how was your college experience like, do you consider yourself a good student, how did you add value or do something different in this role".
A week later, the same recruiter called back and asked when I would like to come on-site for interviews. I chose next week and had a 1 hour tour of the facility with one person, 2 1-hr interviews, lunch, and then 2 more 1-hr interviews. The interviews themselves were not challenging - they mainly look for fit. They actually didn't ask any new questions. I ended up getting the same 2 questions from all four interviewers..."why mcmaster-carr" and "walk me through your resume". I just filled up the rest of the time with questions of my own to them.
The one thing to be aware of is that McMaster is a privately-held company and a lot of details about culture/career/business model are shrouded in secrecy. So you need to be comfortable with ambiguity even with your career progression to work there. I asked a lot of questions about the role/program I was applying to and none of the interviewers could actually answer since they didn't know themselves - I had no real idea of what my careerpath would be like if I joined. The big realization I came to in only the 2nd to last interview was that "Management Development" in McMaster just means Operations and "Systems" just means IT. I was under the impression that the first was a general management rotational program and that the second was a specific rotation within that program. That was completely wrong and they were two separate roles.
A week later I got a letter in the mail dated 4 days after my interview saying that they were going a different route and couldn't offer me the role. I must say that all the people I talked to were very impressive and McMaster only recruits at big name schools. It really just depends on fit and whether they see you meshing in their company culture.