"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.

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 constructive criticism focused on the specifics that you want to discuss, and avoid the temptation to make judgeme...