Hi,

Quant skill is quite a broad definition. Let's split into the following:

1) Fast math - train, train and train again

- Learn how to multiply double digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo)
- Learn the division table up to 1/11 (i.e. 5/6 = 83.3)
- Learn how to work with zeros (Hint: 4000000 = 4*10ˆ6)
- Use math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice

2) Critical reasoning

- GMAT test CR and IR parts (Official guide and Manhattan prep)
- Mckinsey practice tests
- PST like tests from the web

3) Working with tables and graphs and deriving conclusions

- Study "Say it with Charts" book
- Check all available MBB presentations and publications. Practice to derive conclusions and check yourself with the actual ones from the article / presentation
- GMAT IR part (Official guide and Manhattan prep)
- Learn basic statistics (Any GMAT or MBA prep guides)

4) Case math

- Practice common market sizing topics (Airport passenger flow, real estate volumes, subway passenger flows, car usage, etc.). You should become comfortable with making assumptions
- Learn key financial topics (P&L and balance sheet and how to analyze them, Basic Valuation principles via NPV and comps) and case tips (e.g. Rule of 72, Perpetuity)

Good luck!