How to calculate the seasonal performance of an index?

Hi All, I have been looking to reproduce this seasonality chart of S&P500 Index but for NASDAQ100 Index - Imgur: The magic of the Internet

I request help in calculating the seasonal performance (%) of the index, something as simple as mean/median monthly performance will do, e.g., QQQ - Seasonality

Historical daily Close NASDAQ100 Data is here - NASDAQ 100 Index (NASDAQ100) | FRED | St. Louis Fed