Proofreading is the last stage a text undergoes before being published.
In this last stage the text is checked for:

• Grammar
• Syntax
• Spelling
• Punctuation
• Capitalization
• Links and references
• Numbers
• Adherence to a style guide
• Consistency

As you can see, proofreading does not entail rephrasing or changing words: this is done in the editing stage.

Proofreading requires a thorough understanding of the language, style guides, and the ability to apply that knowledge.
This entails double-checking every word and remaining focused from start to finish. This ability to maintain focus comes with practice and cannot be acquired quickly.

It is also, I think, a matter of character since you have to be patient and detail-oriented.

When proofreading translations, checking terms in dictionaries and glossaries is often necessary.

I am very patient, calm, and detailed-oriented, due in part to my character but also as a result of my studies and professional work. I can guarantee your documents will be flawless.