Summarize:

Avoid using "I" statements. These are things like "I don't like how you did this" or "I feel like you are not trying". Instead, use actual facts to support the points you are making. Say, "You have not turned in the past three assignments. That demonstrates a lack of commitment to this class." Leave your personal feelings out of it. Sure, you might feel some negativity towards the other person, or maybe you just don't click. Don't let that inform the criticism that you are giving. Instead of saying, "You're not helping the team" try saying, "It frustrates your colleagues when you fail to show up on time for meetings." Try to avoid giving vague suggestions for improved performance. Something like, "You need to do better" is not really helpful. Instead, talk about the actual thing that needs improvement. Make your feedback actionable. Give the other person tangible things that they need to do. Instead of "You need to help more around the house" try "You need to make your bed and feed the dog every morning before school." Try to give your feedback when it can still be helpful. If you wait until you are completely dissatisfied, there might not be much time left to salvage the situation. If you notice a problem, address it quickly. If given correctly, your feedback could help the other person improve. Don't wait until a student has failed your class to give feedback. Instead, discuss ways they can improve throughout the semester.
Be objective. Focus on the situation instead of the person. Give task specific suggestions. Give timely feedback.