Try to think about the papers in relation to each other. Try to rank them, from strongest to weakest. The only way to be fair is to be consistent. Be as specific and constructive as possible. Identify specific issues or problems in the margins, and then sum up the general strengths and weaknesses in a summary statement.
Negative comments make students feel powerless.
Note the type of problem, mark it once or twice, and note at the end that it distracted you from the content of the paper. Aim not simply to justify the grade you give, but to help the student improve. Suggest strategies for improvement. Point out themes and connections the student has missed. Focus on one or two main problems that you'd like them to work on.