They should address the strengths and weaknesses of the paper and offer suggestions for improvement. The criticism should be constructive and accompanied by positive feedback. Avoid at all costs the use of sarcasm or humour. The humour will likely be misunderstood in the context and, like the sarcasm, be taken as hurtful and the result will be alienation, not a learning experience. Allow the student to seek out the proper form of the identified problems of style.
Many new markers have a tendency to edit everything in the paper.
Seek out the best and worst aspects of the paper for extensive comments and use shorter remarks for lesser notable aspects. The speech on the importance of this task has already been made, so here are some more practical suggestions. After you have assigned a mark, you may want to pile the papers according to the A's, B's, etc. and quickly review each pile. This will assure you that you have been consistent with the grade assigned to a particular quality. Descriptive remarks of each level have been compiled below and may be helpful to you in deciding what constitutes a paper of each grade.