The appointment letter format is not prescribed anywhere. It is a simple letter in which the following things should be mentioned:
1. It should be on the letter head of the company.
2. It should be addressed to CA who is appointed as an auditor.
3. It should mention the date of AGM in which the appointment is made, the relevant section of law under which the appointment is made and most importantly, the "FIRM REGISTRATION NUMBER" of the CA being appointed.
4. It must bear date and place along with the signature of director/ company secretary / any other authorised person.