My parents are both screenwriters. They started asking me for notes as soon as I could read. They give me notes on my writing too. This is what I've learned. Sorry if it's dry as fuck.
  1. Don't be an asshole. Follow the golden rule.
  2. Don't be a kiss-ass. It is not helpful to hear your script is perfect if it could use work.
  3. If they give you pages to read but not a full finished draft, read with enthusiasm and afterwards just say, "It's good. Keep going."
  4. Before you read a full finished draft, ask if there's anything they want you to keep an eye out for.
  5. After you read, let it rest in your mind before you talk.
    If you wait, pattens will emerge and the big picture will become clear.
  6. Give your notes in person or over the phone, not via email.
    Because, as we all know, it is so easy to misread tone in an email. And tone is crucial here.
  7. First, say what you liked first. Be specific and effusive. Let them know via praise that you are on their side.
    Even if you hated it, find something to like.
  8. Now give your big-picture notes. People can only really absorb two or three of these. Be smart and prioritize what you think they need to hear most.
    Organize your thoughts by theme: I don't know what your protagonist wants, the stakes seem to get lower here, etc. Give your friend the headlines and then gently back your points up with details.
  9. Use soft language
    I think, I felt, I didn't quite understand, It might be stronger if, I wanted this to be a little clearer. Ask questions about intention/tone. Let it be a discussion.
  10. Highlight problems, don't offer solutions
    Just be a second pair of eyes. Let them know where you think the issues are. If they want help figuring out how to fix it, they will ask for it.
  11. Then give your page by page notes.
    This should all be small stuff. Typos, clarifications, praise. You can also use this time to show specific examples of the larger issues you already discussed.
  12. If your friend gets defensive about something, back off that point. If they're still defensive, say that the rest of your notes are really small and offer to email them later.
    Prioritize your relationship over this notes session. Always.
  13. Finish by telling them something really positive. Now get the hell out of there and give them their space.
    Sometimes, when my mom is giving me really excellent notes over the phone, I am simultaneously writing down her thoughts and giving her the finger with my other hand. It's hard to hear feed-back. Even good feedback. Even or especially when the other person is right. Get in, get out.
  14. Ps. Stephen King has the best rule I've read on *taking* notes, which is essentially majority rules. If 4 out of 5 readers tell you you have a problem with your ending, then you need to change your ending. If fewer than half think you have a problem, then you win & you get to keep your ending.