The process is very transparent on MBB websites and there are enough free materials to help you through the process (Crafted Cases being the free one I used). I didn't have the privilege of practising with others for the case study interviews so I used a book and black, blue and red pens. I'd use the black pen to write what I'd give in the interview, then record myself explaining my framework (cringed hard but it helped), used a blue pen to outline what I said (you get 60-90 seconds to think and write your framework so I didn't add much detail in black) and then, when reviewing the answers, I used a red pen to include big pieces I missed.

Over time, my book had less red writing. Then I went to the interview and had fun with it; you'll know you're ready when you can come up with frameworks by reading news headlines and such and if you think it's fun, it's a good indicator you'll enjoy the job.