Use standard letter writing conventions in formatting the letter. Write your name and address in the top right of the letter. Below that and to the left of the body, write the date and then the name and address of the person to whom the letter is being written. Open the letter with a standard formal greeting.
Identify your reason for writing the letter. Even if the recipient is expecting a critique letter from you, this helps to frame the content of the letter in a formalized context.
Open the critique with an objective summary of what you are critiquing, be it the person's behavior, a piece of creative work or a business project. In summarizing the object, you indicate to the recipient your understanding of the object. If your summary isn't accurate, your critique could be flawed.
Follow your summary by describing the person's successful achievements in the object being critiqued. For example, you might write, "Your essay on Huey Newton successfully contextualizes the leader in the tumultuous era of radicalism that was the 1960s in America."
Transition from the summary of the positive attributes with a sentence following this format, "In order to build successfully upon the strengths of your project, consider employing the following tactics." By framing your critique as suggestions for improvement, rather than mere identification of weakness, you make the recipient much more eager to consider your critiques.
List your specific critiques in short, bulleted lines starting with an actionable verb. For example, "Practice concision in explaining Newton's history with the Black Panther Party," or "Avoid reductive generalizations in your summary."
Indicate to the recipient your willingness and eagerness to offer further suggestions and advice regarding any and all future versions of the object which you have just critiqued. This will frame your criticism properly in the context of concern and help.
Close your letter according to standard letter writing practice. Use a formal sign-off such as "Respectfully," or "Sincerely," and sign the letter and print your name underneath your signature.