Start with the positive aspects of the work which you are writing a commentary on. You should look for redeeming aspects that provide an opportunity for praise and feel free to offer good remarks about these. This will put the recipient in a good mindset, as she reads the rest of your comments.
Transition from the positive aspects to the negative ones by explaining that the following criticisms represent opportunities for improvement. You can say, "In addition to the good parts, there are a few parts that need some improvement and I would like to offer ways to improve these aspects." If you frame your criticism as an opportunity for improvement, then the recipient of the commentary will interpret your tone as helpful and not mean-spirited.
Present your critical comments in a constructive way. Do not list the poor aspects of something without offering ways that they can be improved. These comments should be unbiased and relevant, such that they actually give the recipient the opportunity to fix problems.
Summarize the relative strengths and weaknesses. This will allow the recipient to determine what areas need improvement and it contextualizes all of your more specific comments.
End with a subtle suggestion,such as, "Doing these things will greatly improve things for everyone." Ending the commentary in this way keeps the tone nice and prevents someone from feeling commanded or insulted.