Dear Sami,
You have asked best way of constructing a memo. In that case, I recommend you drafting memo on your own and uploading it here. Some senior like me will correct it. While drafting, maintain the following sequence:
a) Who prepared the report, on what incident the report was prepared, when you received it
b) Overall what were the contents, what were the recommendations
c) What was the truth, how you discovered the falsification of the report
Up to this point, just write the facts without application of logic.
d) Was there any laxity by the person who prepared the report, did he not take sufficient precautions to dig for the truth
e) What were the consequences of the falsified report, was there damage to the reputation of any person or business entity
f) Lastly come to the warning part. Mention that the falsification of report is misconduct. This warning letter is a punishment for the misrepresentation of facts.
For the each point, make a separate paragraph.
Thanks,
Dinesh Divekar