Stumble where appropriate. Apologize if you stumble inadvertently. Read ALL the paper before giving feedback. After reading, look up and share some positive feedback. Respond to the writer's needs first, address their initial questions, and then you can share your concerns.
Good tutors can always find multiple routes to the same goal.
When you disagree with a student, respond first by paraphrasing what that student has said so what you disagree about is clear. Then suggest some alternative ways of thinking about the problem without rejecting the writer's solution. Ask the student to explain an idea to you if it's unclear in the paper. Be as reassuring as possible. Summarize what they've accomplished, not just at the end, but as you go along.