There is no one-size-fits-all answer to this question, as the best way to become a proofreader may vary depending on your individual skills and experience. However, here are a few general tips that may help you get started:
Read a lot. The more you read, the better your eye will be at spotting errors.
Get familiar with the style guide of your chosen language. This will help you know what is considered “correct” and will make it easier to spot errors.
Pay attention to detail. This is one of the most important qualities for a proofreader.
Practice, practice, practice. The more you practice proofreading, the better you will become at it.
There are also a number of online courses and resources available that can help you learn proofreading skills. Here are a few to check out:
Proofreading Course from the University of Washington
Online Proofreading Training from Proofreading Academy
6-Week Online Proofreading Course from Kibin
Proofreading Tips & Tricks from the NYU Writing Center