I find the least said the best,
I have written a few, all stated I would like to resign my position giving the necessary notice without any added comment, Makes sure you will be given a reference, and gives them a great impression of you, even though they know why you have resigned you kept your dignity.
I remember after one letter, the boss came to speak to me asking are are you leaving because of so and so, and then an outside supervisor came to speak to me asking if it was the boss, I just stated it was time to move on.
Truth it was because of the boss, but so and so did not help, they knew it the boss came to ask because he was scared, the outside superviser wanted something on the boss. What was my odds when my next employee wanted a reference it was from the boss. Very high.
I come out best, left behind people who distrusted each other.