Someone has asked you to vouch for them, and you want to do it properly. The good news is that recommendation letters are one of the most formulaic documents in professional life. The structure barely changes between a job application, a graduate program, and a scholarship, so once you know the shape you can write one in twenty minutes.
Five paragraphs, in this order:
Who you are
Your title, the organisation, the length of the relationship, and in what capacity. This establishes why your opinion counts.A first strength
A first strength with a real example behind it. Name the situation, what they did, and what happened as a result.A second strength
A second strength, same treatment. Two well evidenced strengths beat six asserted ones.Their character
Something about their character or how they work with people. This is what a reader cannot get from a resume, and it is often the paragraph that is remembered.Your closing recommendation
A clear closing recommendation, an offer to answer questions, and your contact details.
Keep it to one page. Readers who assess applications are working through a stack of these, and a second page almost never earns its place.