A letter of recommendation is written by someone who supervised or taught the candidate, and it works through specificity: the relationship, its length, and one or two concrete examples of the person's work. A letter full of adjectives and no incidents reads as a favour; a letter with one memorable example reads as evidence.
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.