The interview process consists of four steps. The first is a phone call with Beth, which is a 30-minute interview where the details of the role are explained to you with a couple of general questions about your background. The second step is an Excel modeling assignment, which you are given three hours to complete. The third step is a case study with Chris, and the fourth step is a final round at their location in Providence (note: not covered!) and is a mix of a larger case study with more Excel. You'll also get the chance to have lunch with the other analysts/associates there.
In terms of what you need to know, basic Excel functions (up to VLOOKUP) and knowing your way around Excel is definitely helpful. Being able to do quick, simple multiplication in your head will also help.
In terms of the experience, I definitely recommend trying your best and going through with the entire process. Beth, Chris, and everyone else at the office were all incredibly nice and intelligent. It's a nice trip to Providence (cute small town). I would describe it as a super fun crash course on niche strategy consulting where you get to have five instructors--a great learning experience and a great experience overall.