I need to significantly improve mental maths for case interviews - what do I need to do?

I need to significantly improve mental maths for case interviews at firms like BCG, Bain, Mckinsey etc. what do I need to do?

Unfortunately I’m not gifted in mental maths and I struggle to visualize numbers.

What do I need to do improve my mental maths so it doesn’t stop me from reaching my career goals. How long will it take. Any suggested books or resources