Could anyone share their prep journey from start to offer?

Some good answers here already. To offer another perspective, below is my case prep journey from start to MBB offer. I'll preface this by noting that I was a non-business major and had zero casing experience at the start of this process, so some portions of this might be overkill for a business student. This could conceivably fit any timeline longer than 3 weeks, but IMO 6-8 weeks would be ideal. By the time I hit my final rounds, I had given roughly 40 cases and received the same.

1. Read Case In Point

• This book is a great (and funny) introduction to casing. Much of the intro is very helpful and the behavioral advice is great. 
• With that said, the cases in Case In Point suck. They are still worth doing (especially if you are starting from zero casing experience) but they are largely 1/10 difficulty and not emblematic of the style of cases that (most) consulting firms give
• The Ivy Case System is a "meh" technique but, generally, a decent starting point

2. Begin Live Practice

• I cannot stress the importance of live practice enough. Live practice is the only way to get better at casing. Casing is intimidating and hard--you will likely suck at the beginning (I certainly did). Put your ego aside and just start. You will be amazed by the pace of your improvement.
• Who should you case with?
  • Ideally, you have a friend or classmate who is also very motivated to land a consulting job. Make this person your "prep partner." Meet as often as you can (depending on your timeline, this could mean daily) and each session should consist of you giving your partner a full case + feedback and then your partner giving you a full case + feedback. Swap the order each session. Make sure that you prep the case you are going to give in advance.
  • If you are unable to find someone you know, my advice is to use the Rocket Blocks forum. There are tons of people looking for consistent case partners. Make sure you pick someone in the same position as yourself (i.e. if you are a college junior applying for generalist consulting, don't pair yourself with a physics PhD who is prepping for an expert consulting gig). WSO and Fishbowl can also be used to find partners
• Who should you not case with?
  • Remember your high school friend's cousin's sister who works at Bain? DO NOT ask her to do a case with you (yet). If you've barely learned how to walk properly, you shouldn't book sprinting lessons with Usain Bolt--master the basics first. Don't waste this person's time while also making a poor first impression.
• Why should I give cases?
  • Well, given a two-person prep approach, this is the only sensible way to do it. However, IMO, giving a case is an incredibly useful exercise--perhaps more helpful than even doing one (especially when you're first starting out). When you know the answer to the case, observing someone else's "good" and "bad" moves (essentially gaining the perspective of an interviewer) is a tremendous learning opportunity.
• Things to look out for:
  • Make sure that you and your prep partner are at the same "level." If you are a beginner, find another beginner.
  • It is possible that you and your prep partner learn casing at different paces. If you quickly reach "intermediate" level and your partner is still a "beginner," don't be afraid to spend a few sessions focusing on their development. If this fails, don't hesitate to find another partner.

3. Read Case Interview Secrets

• Once you've done about 10 cases, read this book. At this point, you are probably "getting the idea" behind casing but are far from hitting "offer-level" performance. This book is an outstanding resource when it comes to understanding why case interviews exist, what interviewers are looking for, and how to perform at an "offer-level."
• Note: the frameworks in this book, while generally good, are overused. This book has been out for a long time and many interviewers, themselves, read it when they were case prepping. If you go into an interview and use the cookie-cutter Business Situation framework, the interviewer will roll their eyes and ding you for it. However, these frameworks are a good starting point and can be used as the building blocks for "custom frameworks."

4. Live Practice with Better Cases

• Remember when I said that the cases in Case In Point suck? Well, fortunately, there is a functionally-unlimited supply of cases that don't suck
• Google "{elite business school name} case book pdf" and pick a few that you like. These PDFs will have dozens of cases that vary in difficulty, interview style, industry, etc., and, in most cases, are "retired" cases that top consulting firms previously used in actual interviews. My personal favorite was UVA Darden's casebook.
• These cases are very emblematic of those you will get from top firms and, in some cases, are even more difficult than what you'll get on interview day. The rest of your case prep should come from business school case books 

5. LOMS (Look Over My Shoulder)

• While you are doing step 4, you should also listen to LOMS. Yes, it is expensive. Yes, it is worth it. LOMS is invaluable when it comes to understanding the minor differences between an interview performance that is "offer-level" and one that is merely "good." When I first started listening to LOMS, I thought that I was performing at an "offer-level" and I was nowhere close. I largely credit LOMS with getting me from a point where I likely would have passed R1 but been cut in R2 to being able to pass the case portion of both rounds with relative ease.
• LOMS is both good and bad because it's audio recordings. Personally, I found it difficult to sit down and focus for 30+ mins on an audio recording without getting sidetracked. Instead, I decided to listen to LOMS during my morning workout instead of music and wrote any insights I had in my notes app. This was a more efficient use of time and helped me focus on the recordings more.

6. Case with New People

• Once you reach a point where you feel that you are consistently performing at an "offer-level," you should reach out to new people and do a case with them. This can be a different friend or classmate who is also case prepping, a current (or former) consultant, paid prep coach, etc.
• Casing with new, unfamiliar people is about as similar as you can get to actual interviews

Final Notes:

• Be coachable. Be willing to accept your mistakes and learn from them. This is the only way that you will break bad habits and form good ones.
• Don't fixate on doing a specific number of cases. Some people might only need 10 cases while others could need 100+. You are likely somewhere in between. While I have included my numbers because they are a helpful objective marker, your individual performance level is the sole indicator that matters
• If you struggle with math, improving this should be your top priority. No matter how good your issue tree, brainstorming, and synthesis are, a math mistake will ruin you. There are enough people who want these jobs and won't mess up.
• Keep casing in between the first and final rounds. You will get feedback after R1 that they will "test for" in the final round. Make sure you correct any issues by practicing your weak spots
• Case prep is pointless if you don't get interviews. Make sure that your cover letter and resume are strong, and network as much as you think will be necessary.
• If you have done dozens of cases but still struggle to consistently build a custom framework/issue tree that is not "cookie cutter," you need to zoom out. Fundamentally, anything can be broken down into its component parts. Start thinking about your regular life and break your decisions down into MECE issue trees. For example, if you want to decide on what restaurant to go to, you could divide it into 3 buckets: price, proximity, and food (quality and portion size).  Or maybe what book to buy at a bookstore? -> length, price, genre. You get the idea. The more you practice doing this, the more tailored (and, generally, better) your case issue trees will become.
• Last but not least: the case interview is only 50% of getting an offer. Do not underestimate the importance of the behavioral/fit portion. I know several people who objectively cased better than me but did not have as much success. Once you are at an "offer-level," doing additional cases is a poor use of time. Do one case every few days to stay sharp and focus on behaviorals