Speaking of criticism, even when you know how to make it constructive, it’s easy to get caught up and go too far in one direction or the other. But if you focus on the purpose of the feedback – to help the author improve the story – then, it’s easier to stay on track. Taking writing classes can help with that.
In a novel-writing class, for example, the writing circle is as much about learning to give good feedback as it is to improve everyone’s writing. If you’re bad at giving feedback, the teacher will try to guide and correct your criticism tactics – usually by enforcing guidelines that emphasize the two major parts to a constructive crit – what you say and how you say it.
As far as what you say, it’s a lot like job review training. Start with a positive comment and follow up with something the person “needs to work on.” As you continue, try to keep a balance between positive and negative comments if you can – especially if you’re going around a circle. A good goal is to give one positive comment and one negative comment. In class, you’ll be required to give feedback, and most teachers will list their requirements that way: say x number of things the person did well and x number of things he/she needs to work on.
That minimizes emotional damage to other fledgling writers and teaches each student to give better feedback at the same time (would that other disciplines might emulate this!).
If you’re worried about hurting feelings, a lot of that is dependent on how you say it. Insults and hostile attitudes are strictly taboo (they don’t help and completely miss the point of being constructive.). Instead, you will learn to give specific examples of problems you encountered in a neutral or analytical tone (be polite). Avoid words with harsh or especially strong connotations where possible, and it can also help to give some reasons why you had that reaction so that the writer can judge how much of the issue is a personal reaction and how much is a problem in the story.
Do not try to tell them how to fix the problem unless they ask for suggestions, and even then be careful (try not to push your vision onto the story!).
If you go to a website like Meetup and find a writing circle on your own, they may or may not be strict about how to give feedback. If you break any of the taboos, you may get a warning, or you may not be welcome anymore (some groups are more forgiving than others). You’ll definitely make a horrible first impression (Hi! I’m Joe, and your writing sucks! [Don’t be like Joe!]).
If you follow the rules listed here and in “Keeping ‘Constructive’ in Criticism,” you should be fine, but if you’re at all uncertain, take a novel-writing class and give yourself a chance to practice.