Most people have had at least one boss that they disliked or felt was not doing the job he was hired to do. These situations are uncomfortable and precarious because if you don't say anything, the situation could become worse but if you do offer criticism, your boss is likely to be offended and you could face repercussions at work. If you feel that you must state your case, do so in a professional way that does not accuse but offers constructive advice.
Type "Dear Mr./Ms. (last name)" to open the letter, followed by a colon or a comma. Do not bother with the standard letter format or company letterhead because your boss might interpret the formality as an attempt to make a play for power or believe that you are documenting your letters to make a case to have her fired. Keep the letter informal and sincere because she is more likely to read it and accept the advice in the spirit it was meant.
Explain your intentions and why you are writing the letter. If you do not want the boss to have hard feelings, state that and tell him the advice is meant to be constructive, not accusatory. For instance, you might write "I am writing this letter because it has come to my attention that several employees are deeply upset by some of the events in the workplace and they blame you. I am not writing this letter to accuse you of anything, but rather to inform you that if you don't take action there could be serious problems in our workplace."
Relate the specific events, including the boss's actions, and explain the employees' reaction to these actions. For example, "When you fired Sarah many employees were worried and upset because she didn't receive a written warning first, as outlined in our employee handbook. It is not my place to tell you what to do or that you must fix the situation, but some employees have gone to the district manager with this matter and I worry that more firings could result from the fallout from this problem." Phrase your concerns as workplace concerns instead of making a direct accusation. If your boss is intelligent, she will understand what you are trying to tell her.
Thank your boss for listening, and apologize for potentially overstepping by bringing these matters to his attention. Reiterate your concern for the work environment and express confidence that he can rectify the situation.
Type "Sincerely," and sign your name. Place the letter in your boss's inbox or give it to her privately to reinforce your claim that the letter is a private expression of concern.