This short, useful "How to write a memo" post from Scribendi sums it up nicely: "The memorandum (or memo) is an incredibly versatile form of communication, often used in business settings. In practice, memos answer questions and give information." The post encourages writers to get clear on "intent and audience" (sound familiar?).
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An informative chapter on memo writing, with good tips, from this excellent open textbook on Technical Writing for the workplace
An overview from Purdue's OWL, including this sample memo, useful for formatting
A detailed memo about how to write a memo using appropriate document design
A PDF sample memo from the University of Oklahoma