A professional reference should cover the reporting relationship, the scope of what they owned, and a measurable result. Confirm with the employee what the role needs emphasised before you write.
Someone who supervised your work directly and can give specific examples. A junior lecturer who taught you closely writes a far stronger letter than a department head who barely knows you.
One page, three to four paragraphs. Longer letters dilute the specific evidence that makes them persuasive.
It happens, and some recommenders ask for a draft. The recommender must review, edit and sign it in their own voice — the letter has to be genuinely theirs.