I just want to express my appreciation to my tutor who spent her private time to help me with my writing assignment. Can I say what you did is much more than I thought? Because her work is really very detailed and useful.
The following URL might help you: How to Write a Letter to Your Teacher (with Sample Letters)
Speak from your heart and your thank you letter will be appreciated by your tutor!
If you post your letter we will be happy to take a look at it and offer suggestions!