This one's a writing exercise that doesn't involve much actual writing, at least from your point of view. But it's an important skill to develop.

Alpha readers are people who read your incomplete, unpolished not-working-yet drafts, and make suggestions. They're a special kind of reader - the kind who can see past the use of cliches and the dodgy bit where you've used the same sentence structure four times in a row and the fact that 'inflammable' doesn't mean what you think it does. They're people who can ignore the fact that what you're trying to do might not be their type of story, and might not be how they would have done it. Not everyone needs them, but they're invaluable to those who do.

You will need: a sense of tact and diplomacy, a friend who has some rough drafts they will permit you to read, said rough drafts, a box to put your ego in for a moment.

First, put your ego in the box, and leave it outside the door. It has no place here: this is not about you. (Other than the fact that you'll be learning how to see past the surface of writing to the bones underneath, and what the writer is trying to do.) Nobody cares how you would have written this, or if you don't like space giraffes. All that goes in the box outside.

Take the piece away from your friend. Having them staring at you while you read it will totally distract you from what you're trying to do. Gently send them on their way for about thrice the time you think it will take to read it.

Next, read the piece. Do not have a pen in your hand. Do not correct typos, spelling mistakes, grammar gaffes, word confusion or undulating sentence structure. That's not your job right now.

Think. Hard. What is the piece about? What are the main conflicts, resolutions, character alterations - what, in short, is going on in this story. If this is a novel, it's going to take you a while. It's probably a good idea to tackle it chapter-by-chapter. Write them down. If you have a particularly ascerbic natural tone or you tend to think in a sarcastic manner, it's probably best to plan time to rewrite this stuff later.

Now think even further - look at what you've just noted down, and your gut feel of the story - what do you think the writer was trying to do? Nevermind if they achieved it, or if you don't like it or don't agree with it, what were they trying to do? What did they want to bring across? This is a tricky step - it's the crucial skill of the alpha reader, one that takes some practice, hard thought, and a complete lack of ego.

Some people here like to talk to the writer, and see if what they think they're trying to do marries with what the writer thinks they're trying to do. Personally, I prefer to only talk at the end. Sometimes the writer might think they're trying to do one thing, when they're really trying to do another, or may not yet know, or may not want to talk about things that way. It's up to you.

Now, armed with your knowledge of the structure and intention of the story (as you see it) read the story again. What is and isn't working for their intention? What's missing? What's undermining it? What is working really well (it's equally important to tell them what they need to keep as it is what they need to lose). Remember, your ego is still in the box - this is not about how you would write it. This is about making a story that does what they want it to do.

Make notes on this. If necessary, rewrite things into a tactful response. You'll need to give them all the notes - the conflicts and arcs (because they may not see them, or may think there are some there that haven't actually come through), your views on the intention, your notes on how to strengthen that intention. Keep a copy of these notes.

Hand them to your friend, and scamper to safety while they read it. Be prepared to have a conversation where they explain with anger and dispair that that wasn't what they intended at all, and you completely read it wrong, and it works fine just the way it is! Don't judge them too harshly - when we ask for feedback, every one of us secretly wants to be told that our baby is absolutely perfect and shouldn't be touched, even if we know that's a load of pants.

You may find at a later date they come back to you with questions, hoping for a discussion to illuminate some finer points. Refer to your copy of the notes to refresh, and have that discussion. You may need to rethink your suggestions if your reading of their intention was actually inaccurate (if it's that out of whack, though, then there are big problems with the story - generally that the author is trying to trick themselves, or put too many things in).

That's the job of the alpha reader - not to judge or rewrite, but try to help the writer achieve what they want to with a story.