Hello!
The best is to practice, practice, practice! I could recommend you to start with:
1. GMAT exams: that can be found for free in the internet (GMAT prep with The Economist, London Business School free trial, etc.)
2. Math tools: such as Mimir math for iOS or Math tool on Viktor Cheng´s website)
3. Multiplying any two 2 digit number in 3 seconds: check this great video > https://www.youtube.com/watch?v=6ndkkPZYJHo
Best,
Clara