1) Determine whether it's your place to step in. Do you have a dog in this fight, so to speak? Does it affect you, and were you asked for your imput? If the answer to both is 'No', it might be a good idea to bite your tongue.
2)Be polite. You catch more flies with honey than vinegar.
3) Be compassionate. If you don't like being told something you've made/believe to be true 'su-ks', then it will probably upset the other person.
4) That said, be honest.
5) Respect the fact that others will disagree with you, and you might not be able to change their minds.
6) Give concrete reasons. Saying X thing is terrible is much less helpful than " I feel as though X isn't working for me and this is why."
7) Understand that most everyone has a 'hot button'. Common ones include religion, politics and the way people raise their children. It's very likely a bad idea to involve yourself in this aspect of other people's lives unless clearly and specifically asked to. ( The sole exception being if you have a reasonable suspicion children are being abused or mistreated. That changes things).
8)When giving critique of someone's creation, keep in mind that taste is highly subjective. You might think X is the worst book ever written/picture painted/movie filmed, but that doesn't make it objectively bad.
9) Know when to bow out. Everyone is entitled to an opinion, no matter how clearly wrong it seems to others. Sometimes arguments reach stalemates and it's better to agree to disagree.