I interned at ATK a while back. I would start a 1 week practice schedule and stick to it - don't fret, it's very doable!
A few key principles to your practice plan that I would suggest:
- Prioritize the case - it is much more important than behavioral. 80% case prep, 20% behavioral. Behavioral becomes more important in later rounds.
- Practice structured thinking - it is the single most common pitfall for candidates. Depending on your level, market sizing cases are a great way to start, they force you to be logical, structured the whole way through.
- Nothing beats live practice - start scheduling live practice sessions. I would recommend ~3-5 with peers to get the hang of it. Then at least 1 mock interview with a professional to get targeted feedback, followed by 2-3 more peer practices to consolidate the feedback.
There are also plenty of free online resources (PrepLounge, Victor Cheng, consultingprep101 etc) - good luck!