The first step, once selected, is to take a set of aptitude tests. I was not aware nor prepared for the second phase of this multi-hour long process. The thing I was most unaware and certainly unprepared for is the CCAT section. This is a difficult 50 question, 15 minute test requiring puzzle patterns, mathematical word problems, and grammar related question. It is nearly impossible to complete all 50 questions in the 15 minutes they allow the applicant, so my strong recommendation would be to do some research online and practice, practice, practice for the CCAT. You are given two chances to take this exam; the first is normal and just you alone online. However, the second is proctored via video both on your phone and computer so that they can watch you take the test and watch as your phone points at the keyboard itself.
I wanted to share this information specifically, because had I known this was something I needed and could have prepared for, I would have.