Dear Mksuma,
First things first, you have to place an advertisement in the newspaper to this effect. Alternatively, you may be required to obtain a duly attested affidavit by a First Class Magistrate in this regard.
You may start the letter as follows:
BY REGISTERED POST/COURIER/BY HAND
Dated
The Principal
Subject: Intimation about loss of certificate and request for the issuance of duplicate Certificates.
Sir/Madam,
I was a student in your esteemed institution in the class _____________ from _________ to _________________, with roll number/enrollment number __________. Due to circumstances beyond my control, I have lost/misplaced my qualification certificates (please mention numbers and dates if any), hence this application.
Kindly approve the request to issue the duplicates to the concerned person, enabling me to obtain them as soon as possible.
Thank you.
Best Regards,
MKSUMA
Hope this meets the requirement.