Memo:

A memo is a document that is used to explain changes, address issues, report situations, etc. It is not too long and not entirely short either. It should be concise and to the point whilst maintains all the necessary information needed to convey their message. 

First you label your audience

Then you select a fitting title for the report

Then you write the opening 

Address the main concern

List all key points

Then summarize

Then close with contact information .