The first paragraph of the letter should explain how you know the student.
The second paragraph should include what you know about the student. Are they punctual? Do they have good attention to detail? Are they good at communicating?
Paragraph three should include specific examples of the student exhibiting the skills from paragraph two. You can even compare them to people you have worked with in your career.
In your closing statement mention your belief that the student (by name) will be an outstanding grad student or addition to the company. Provide your number and email with a mention of your eagerness to share the additional perspectives of the candidate.