First, have a read here for the mindset you need to have coming into this: https://www.preplounge.com/en/articles/how-to-shift-your-mindset-to-ace-the-case
Now, you're going to get some very drawn out answers here. That said, time is your biggest problem here. Your interview is coming soon (your runway is shorter than it should be) and you're busy. As such, quite genuinely, the main thing you need to do is optimize time. This is hard if you go it alone (you will naturally be inefficient, learn things the hard way, learn bad habits, etc.). My number 1 advice is to use a coach as a time lever (especially since you have a job and can afford it).
In general how can you prepare? (General Advice)
1) An initial planning session with a coach: 1 hour with a coach now will have a productivity multiplier effect on all your efforts moving forward. They will figure out what materials are best for you, guide you towards the best ways to learn, and come up with a preparation plan with you.
2) Leverage free resources first: PrepLounge Q&A and case library, Poets and Quants, SpencerTom, Google, etc.). Leverage these options, read-up, and build that casingknowledge
Importantly, read The Economist, the Financial Times, McKinsey Insights, and BCG Insights daily to build up your business knowledge.
3) Case with other PrepLoungers: Casing with other PrepLoungers is free. Not only do you get to practice casing, but you get direct feedback. Additionally, you learn a lot just from casing others. Finally, from other PrepLoungers you'll learn which materials/coaches are helpful.