Only what you personally saw
Write about the worker's quality, reliability, communication, safety and timeliness based on your own experience working with them. You will confirm this declaration before sending.
Be specific
Mention the kind of projects, the period you worked together, and the skills you observed.
The worker decides
The worker can publish it, keep it private, or decline it. They cannot change your words, and you should not ask them to write it for you.