Reading and critiquing the drafts of others will help you in critiquing your own writing. Revision is easier if we have feedback, comments and observations from others, to consider. Even if we ultimately decide to reject particular pieces of advice, we become more aware of what we're trying to say and how we're trying to present it.
Three general guidelines should govern your comments about a draft:
1. Be specific and thorough. Saying something is "good" or "interesting" or "unclear" isn't very helpful--take the time to explain why it's good or interesting or unclear. Similarly, if you think the evidence is weak, explain why and suggest ways to strengthen it; if a paragraph rambles, tell the writer what she should do about it (divide the paragraph into several paragraphs? eliminate some of the paragraph? incorporate the material into other paragraphs?), etc. Remember, you're trying to help, not judge: constructive criticism coupled with positive suggestions for revision will help the writer most. (Students complain about not getting enough feedback, but they never complain about getting too much.)
2. Don't focus on editing issues like spelling, grammar, typos. Instead, give attention to the quality of the ideas expressed, how well they're supported, whether the paper is convincing, how the paper is structured, the clarity and sophistication of the thesis, sentence and paragraph development, etc. Note recurring grammatical or stylistic problems, in other words, but don't dwell on them.
3. Be both constructive and critical. Look for things that are good and for those that need improvement. It's as important for the writer to know what's working as it is for her to know what isn't. But this isn't a love fest, and the writer will benefit more from thoughtful criticism and specific suggestions than a string of compliments. Just keep in mind that being critical doesn't mean being hostile or sarcastic like a movie reviewer.
Here are specific steps for writing a critique of a draft:
1. Read the draft all the way through to get a feel for the argument. Don't make any comments during this first reading.
2. Go back to the beginning and start reading again. Stop at the end of each paragraph to consider a) the unity of the paragraph (does it stick to one topic?); b) the coherence of the paragraph (does the paragraph move smoothly from one sentence to the next?); c) the development of the paragraph (is the supporting evidence specific and concrete? is there enough evidence? does the evidence support the main idea?); d) the paragraph's place in the organization and development of the paper as a whole. Comment on any area that seems to you to be well done or to need work. If you believe something should be cut or changed, be specific about what you think should be done and why.
3. At the end of the paper, do two things. First, describe what you see as the paper's two or three biggest strengths and two or three most important areas for improvement. Be as specific as possible. Second, write an assessment of the paper and its argument as a whole. In particular, compare the thesis announced at the beginning with what the paper has actually proven. Drafts often develop a different argument from the one they start out with; at the very least they usually become more specific and precise. Tell the reader what argument you think the paper has made. Assess this argument: does it follow the assignment and is it a) clear b) specific and c) non-trivial, i.e. does it articulate a position on which there could be disagreement? Make detailed and specific revision suggestions in light of this assessment that you believe will make the paper stronger; focus on such issues as the evidence, the structure of the paper, points that seem to be missing or unnecessary, etc. This is a vital step, one that requires you to be at your most thoughtful and honest. If the writer has proven something obvious, or has not been clear, or has provided poor evidence for a key point, she needs to be told. Say it nicely, but say it--otherwise you're not being helpful.