Greetings!
A memo generally starts with a heading, like this:
TO:
FROM:
DATE:
SUBJECT:
After that, you would address the purpose of your memo: "I am writing to thank you for providing me with feedback on my essay."
Memos are often used in business to inform employees or members about a particular issue or to request someone's help with something. A "thank you memo" is not something I've run across before. Just keep it short and to the point and very polite, and you should be fine.
Feedback can be tricky, as I well know! You want to point out errors without stomping on someone's feelings. Always find something positive to say. You can soften criticism with phrases like "consider re-writing this sentence..." or "be wary of ..." or "Try to [do this] more..." Put yourself in the reader's place, while remembering that some people are more sensitive than others, so err on the side of caution.
I hope this helps!
Thanks,
Sarah, EssayForum.com
Sarah, EssayForum.com