First of all, you must always consider feelings of the person who is at the receiving end of your criticism. Put yourself in his/her shoes. This will give you a clearer picture of how you would feel, if someone said the same things to you.
Do not point out someone’s mistakes right from the beginning as this will only discourage them. Before you plunge on to their weaknesses, highlight their stronger areas and accomplishments. Build them as a person, before you start pointing out areas where they have been struggling.
If you have criticised someone over something, always remain prepared to provide any help that they need in order to improve. Commit yourself to the fact that you want to help them improve and don’t back down.
Always speak the truth which comes from your heart. Try and understand the fact that everyone wants to grow and improve himself/herself and they need time to do that.
Once you are giving constructive criticism, the other person must be allowed to ask you any questions and you should be kind enough to answer them all. There is a fine line between criticizing someone and insulting someone, make sure you do not cross that line. Respect him/her and allow them to comment on whatever you are putting on the table.
Try not to get involved in an argument. Try to explain to the other person how you are just saying things which you have felt, heard or seen, out of sheer sincerity.
Once you are done, you should thank the other person for the respect they have offered you just by listening to what you have to say.