Not theory, the exact sequence that works, in order. My own numbers and where this comes from are at the bottom of the page, if you want the proof.
You don't need to know anything about case interviews to start. Here's the exact order that works, follow it in sequence rather than skipping ahead.
Before doing anything else, look at Bain's own case interview page just to see what a real case actually looks like, and the level you're expected to perform at. Don't skip this step. It's the difference between practising blind and practising toward something.
Then Crafting Cases' free 7-day course. The single biggest unlock early on: good structured exposure to how cases actually work, for free.
CaseCoach next: more practice reps and a few different angles on the same fundamentals, still free.
Go back to Bain's page (or whichever firm you're applying to, most publish something similar) and run their real sample cases.
The first time you'll feel the difference between reading about structure and actually holding one out loud under mild pressure.
Someone from your network if you've got one. Otherwise, this is the one thing I'd recommend paying for. It's the step that's hardest to replicate for free.
My own mock case recording is coming to this page once it's edited, so you can see exactly what step 4 looks like end to end before you try it yourself.
Started scattergun in year one: quantity over quality, generic CVs, stuck at online tests with no idea why. Locking in on consulting specifically in second year changed everything, tailored CVs, serious case prep, and it converted. 170 applications, 17 assessment centres, 4 offers: two in consulting (OC&C, Deloitte), one private equity (Carlyle), one banking (HSBC). Took up the OC&C offer and have now fully pivoted into consulting, that's the credibility behind everything on this page.