Take in-line critiques for example: critiques in which the critiquer's words are inserted directly into the writing. The critiquer's words are usually bright red or have a green background, anything to distinguish it from the writing itself. By writing these critiques you are forced not only to search for errors and shoot for clarity, but also to write down WHY it is an error or WHY it is unclear. This is key.
I am currently on a critique binge. I am writing hundreds of words of critique in order to bring my knowledge of diction, punctuation, etc. to the surface so that I can use it. I feel this honing of my skills is a major step in entering the field of writing.