How to write a memo to an employee for a mistake committed by employee

As a HR what should I do if supervisor ask me to issue memo to particular employee

Hi,

Gather more information from the Supervisor about the employee and the reason which forced the Supervisor to recommend for Memo. Analyze the gravity of issue whether it demands warning/memo or verbal warning. Check the employee's personal file for any related past issues.

In case the nature of the issue is trivial call the employee and warn him and take some apology letter from the employee. However if the nature of misconduct is quality or disciplinary related with more significance then issue a warning letter to the employee. Also always take the recommendation of the Supervisor in writing through email or written communication.

Advice memos/warnings are the prerogative of the supervisor who is controlling the employee. It is seen that the unpleasant task of disciplining a subordinate is left to HR while pleasant duties like promotion, leave sanction, probation confirmation, and other administrative orders, etc are issued by the controlling authorities.
If the present matter is also being dealt like that, you should resist it and tell the supervisor concerned that it is his task and you can only help by drafting the required communication.