|courtesy of Vidalia_11|
1. Find out where the piece is. Has it been edited only for grammar or is this the 6th version? Also, is there anything the author wants you to pay special attention to?
Line edits are great, but if your partners is still fleshing out major plot points, it's not what they need. A quick check to find out where a piece stands can help both of you.
2. Be specific! Don't just say "this is nice" or "this needs work". Say," this run-on sentence makes it hard to understand the point" or "this description makes me feel like I'm there". The more information you can give, the better.
3. Tact. Don't be afraid to give honest feedback, but be mindful of how you'd like to have this presented to you. It helps if you always share the critique as just your opinion. Also, no matter how weak a piece is, you should make it a priority to point out at least a few things you like about it.
4. Be cautious of offering alternative options for sentences or words. Some people will love to hear different ways to word things, but some will take this as you trying to rewrite the piece for them. It's best to avoid this until/unless you know how it will be seen by the author.
5. Critiquing is subjective, but remember the subject isn't what you're commenting on. Leprechauns might not be your favorite magical creature, but someone's fantasy novel featuring a band of magical leprechauns is not the place to discuss it. However, should a leprechaun show up in the middle of your partners very non-magical legal thriller, you might want to suggest they've gone a bit outside their genre :)
Just like writing, critiquing is a skill. The more you do it, the better you'll get.