it's a 1 hour written case. I would use the following approach to prepare:
1) Prepare for a regular case interview - it helps a lot. Basically, prep lounge website is about it
2) Check if the calculator is allowed. If not - you have to train mental math. I posted the main tips here: https://www.preplounge.com/en/consulting-forum/tips-to-do-big-multiplications-in-my-mind-726#a1422
3) Practice doing slides.
- Study "Say it with Charts" book by Barbara Minto
- Check all available MBB presentations and publications. Practice deriving conclusions from the slides. You can find tons of them (https://www.slideshare.net/search/slideshow?lang=%2A%2A&page=1&q=mckinsey+presentation&qid=d321c385-1cbc-4dfc-8b8e-d91bf553784e&searchfrom=header&sort=relevance)
- Try to reproduce their slides in PPT / on paper. Check yourself by trying to make a slide on any given business article.
4) Practice reading cases fast and prioritizing the information. I found several useful sources:
- Written cases you'll be able to find in google or in case books. I've seen a couple in "Vault Guide to the Case Interview" and "Insead Business Admission Test"
- Harvard cases - either buy or try to find online. You can find a couple of MIT cases here for free: https://mitsloan.mit.edu/LearningEdge/Pages/Case-Studies.aspx Unfortunately free cases don't have the prep questions.
5) Practice written cases. For example, try a BCG Wine in a Basket practice case https://www.dropbox.com/sh/zor4m49eyx5qxal/AABeUN6mtiGkWhEklRjszX2Oa?dl=0