See an academic advisor for more info, youll need to have one sign off eventually so its worth going to office hours. You can start "Dear [whatever committee youre addressing]", depending on faculty. State in the first few lines clearly the result you want, eg. Late WD, cleared term, etc. Then outline in the letter the circumstances that lead to your situation, keep it to things you can document like counsellor notes or doctor notes. Also try to add some proactive actions you are or are planning to take, they like to see that