The memorandum should be written as follows:
This will inform the addressee as to who sends the memo.
This is for whom the memorandum is written for.
In the next part, you should explain for what reason the memo is written. This can be for various reasons. For instance, there could be a memo sent to all the staffs of a company regarding the changes made in the company car park. Management can also send memo about any visitors arriving at the company.
Finally, when you are ending the memo, close it with date of the event. Memorandum is also used as a legal document.