1. Start by complimenting their positive qualities. This will help to build rapport and make the person more receptive to what you have to say. For example, you could say, "I love your enthusiasm and passion for singing. You have a lot of potential."
2. Be specific about what needs improvement. Don't just say, "You sing badly." Instead, provide specific examples of what you think they could work on. For example, you could say, "Your pitch is a bit off, and you could work on your breathing."
3. Offer to help them improve. If the person is willing, offer to help them improve their singing. You could give them some tips or resources, or even offer to take them to a singing lesson.
4. Be respectful and supportive. Remember that this is a difficult conversation for both of you. Be respectful of the person's feelings and be supportive of their efforts to improve.
5. Don't be afraid to be direct. There is no point in sugarcoating it too much if someone cannot sing. At the end of the day everyone has different talents and just being honest and polite will suffice.