Two things about constructive criticism:
Unless the person has specifically asked for it beforehand, you should always ask before giving it.
For it to actually be constructive, you have to be able to offer at least a potential solution to the problem. The goal of constructive criticism is to be helpful. “X is wrong :)” is NOT helpful, so maybe try something more along the lines of “X seems a bit out of place, maybe if you shifted it a bit, or tried Y”.
Also, a third thing I feel the need to point out, there is a BIG difference between someone else’s work not being to your personal preference, and it actually needing to be changed or fixed.
Another thing I’ve learned from my college classes is that it’s equally important to point out what the person did RIGHT. It allows them to understand both their strengths and weaknesses and also makes it feel less like a personal attack.