Keep your criticism to your observations, and the impact they have. Don't try to fix the problem, just identify it.

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.

"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.