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.
Have feedback but you are not sure how to put it across? Here are 6 helpful tips to provide constructive criticism that's tactful and helpful at the same time. Tip 1) Use the Feedback Sandwich. Tip 2) Focus on the situation, not the person. Tip 3) Be specific with your feedback.
To help people improve talk about things they can do something about, rather than those out of their control. Critiquing the former makes your criticism constructive; critiquing the latter makes the person feel bad as they can’t do anything about it, even if they want to.
Understand the person’s situation and his/her objectives, then provide your critique based on that. And if you need to talk about something out of their control, balance it out by talking about things they can control.
Give recommendations on what the person can do to improve so they have a clear idea of what you have in mind and get a strong call-to-action.
With your recommendations, (a) be specific with your suggestions and (b) briefly explain the rationale behind the recommendation. Also, try to limit examples to one per point to make your case more impactful.
Can you give someone criticism without hurting their feelings or making them angry? Can you do it kindly? I think that's a difficult proposition for most people, but in truth it's possible to give criticism with kindness and have a decent chance of having the person take it constructively.
When you know someone can be really defensive, it makes giving them any sort of constructive criticism a battle you'd probably rather avoid. If something needs to be said, it all depends on the clarity of your message and how you communicate it.