YOU NEED A CRITIQUE GROUP!
If you haven't seen that little bit of advice as soon as you punched "how to publish a book" into google, you must have spelled it wrong. The thing is, not many people I've found have recommended what to do once you get a critique group, not to mention when you have someone else's hard work in front of you...what the heck do you do with it!? Here's some ideas to get you started:
crit-a person made of awesome with a side of grammar, a splash of expert reader, and a sprinkling of hero
1. Ask your writer what he/she wants you to answer. Critiquing grammar, spelling, etc., the mechanics of writing, is a given but what's really concerning your writer? Do they want you to rip their work to shreds? Are they worried about the main character being an unlikable putz? The climax not being climaxy enough? That metaphor a bit too ambiguous? Answer that questions honestly and you've already become the best crit a gal could have.
2. Read their work like a reader. If you're a writer then by default you're also a reader. You've read dozens if not hundreds or thousands of books so in a way, you're an expert there. After I get a critique back from a partner I look first at their comments from reading the, "Oh, this is funny!" or "Gaaaaahhhhh, DON'T GO IN THE BLACK CAVE!!!" These tidbits tell the writer that his/her writing is spot on and accomplishing the emotions they intended in the reader. They are so so so so valuable. Did you get that? Soooooooooo valuable. Read the book, every time you have a reaction to their words comment about it.
3. Mark the following: A) places you had to read twice to understand the writer's meaning B) Anything that seemed unrealistic or odd for no reason at all. C) Plot holes
plot hole-that gaping abyss where what your character is/does is different than what should be possible or obvious given circumstances you described previously (Bob snuggles with a cat he just adopted-page 101. Only, see, the thing is, page 4 said he was deathly allergic to cats....dun, dun, dun, PLOT HOLE!)
4. Be positive, but stay honest as you sum up your thoughts on their work. Start with good things before you jump into the problems and if there are a lot of issues, start with the biggest first. There's a good chance they will see the smaller ones as they fix the bigger ones.
5. Read the work again. The first time you are look you're experiencing their world for the first time and curiosity often makes you miss errors. This is also the time to make suggestions like taking out useless paragraph after paragraph of description or that back-story that really doesn't have a purpose. You might even be able to suggest going more in depth in certain areas.
I'm sure there are other methods but these 5 steps of critiquing can help you get started. Being a critique partner or in a group is one of the best ways to improve your writing. You'll spot everything faster and be able to fix it in your own work, so don't let the worry of doing it wrong stop you.