Readers of recommendation letters see the same praise in every one. What persuades them is evidence: how you know the person, what you watched them do, and how they compare with others you have worked with.
Your relationship.
Your role, how long you have known them, and in what capacity you supervised or taught them.
Two specific examples.
A project delivered, a problem solved, a result achieved. Include numbers if you have them.
The qualities those examples show.
Judgment, reliability, initiative. Name the ones the opportunity calls for.
A clear endorsement.
Say plainly that you recommend them and that you are willing to be contacted.