Warning letter is written to warn the person for wrong action. If the case is severe the letter can also be considered as legal.
This letter is to warn the person before the actual action taken. The reason of warning should be mention in the letter and what is expected by the reader should be written. Action should be described in the letter.
The warning letter should provide a time period within which this action must be rectified, after which there will be inevitable steps taken by the party issuing the letter.
The letter should be written in authorative tone and needs mention clear what is expected by the reader.