"Sandwiching" your critique between two positive things about the person's softens the blow, and avoids it coming off like an attack. The mix of positive and negative makes people more likely to pay attention to the whole package.
Instead of being snarky and vague, explain why you think your criticism is valid and be specific and constructive about what you think would be an improvement. The former doesn’t inform much and makes people unhappy; the latter at least gives some ideas for improvement.
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Keep your criticism to your observations, and the impact they have. Don't try to fix the problem, just identify it.
Offer to help fix the problem, and to support the solution that the other person comes up with. Unless you know how to do the work your coworker is doing, don't try to solve it for them—they'll ignore your feedback and you.
The point of your criticism is to help someone improve, or to correct a problem, and your feedbacks should carry that message. If you’re doing anything but that, reevaluate whether you actually have legitimate criticism to give, or you just need to talk to someone.
Offer positive and specific suggestions to alleviate the issue at hand, or identify the problem clearly without talking about the person, just the issue.
Criticism by nature can be personal, but delivering it you need to separate your thoughts on someone's work or behavior from what you think of them.
Keep your criticism focused on the specifics that you want to discuss, and avoid the temptation to make judgements of the person or their work based on the specific feedback you want to give. Remember, "you need to respond to urgent issues faster" is not the same as "you're slow." You want to communicate the former, not the latter.
The more specific your feedback, the more actionable it is for the one receiving it. Example: Asking for an article on communication is vague while asking for one on public speaking is specific.
An specific feedback that doesn’t target the person is easier to understand and act upon.