What kind of letter should you write for your old teacher that is retiring so she knows you appreciated her?

Write about how much she helped you when you were in her class and how she made a difference in your life. Write kind thoughts about her so she will feel like she's done what she was set out to do. I understand the situation kind of the same. My teacher has only been teaching in my school for one year, and she has been a great teacher, but she is getting fired. Of course, that isn't the same thing, but still....