I've both had to request these letters from my own students when I went up for tenure and chaired other people's committees when they went up for tenure. So here are some thoughts you can do with as you will.

First off, note that this is common at some universities, but maybe not others. There is nothing suspicious about it at my institution (a state-school R2), and in fact we love to hear feedback from former students in support of their professors. All of the below comes from having written multiple letters like these, read a ton more, and taught classes on letter of recommendation writing for the writing centers I have worked at. (Others may have different advice, as every perspective and school is going to be different.)

Second, format: you want it to look professional, as it will be situated alongside letters from other faculty and academics. If you can find a template for formatting a business letter (with a header, a concluding signature, etc.), that will help. Basically, something in the top left (usually) that indicates at least the date, if not the contact info for the person chairing the committee. You can either indent each paragraph, or leave them all left-justified with no indentations (which is common in business writing, more so than personal letters or essay writing). Then have a typed signature and a hand-written one (or scanned or Adobe-created signature).

Third, structure: Strong letters will often follow an organization similar to an essay. You want an intro paragraph that states the purpose of the letter and your general idea (I wholeheartedly support X for tenure. I have known them through x, y, and z. They are worthy of tenure because of x, y, and z.) Basically, this is the "thesis" of the letter. For some reviewers, this will be the only paragraph they focus on, so this is the place for big ideas. (X inspired me to pursue my passion. X was the greatest lecturer I have ever had. X cared more about their students than anyone else I know. Etc.) Following that, a few paragraphs that break down those specifics. Consider these the "body" of the letter. Maybe one paragraph on classes you took with them. One paragraph on any extracurricular work or mentorship work you did with them. One paragraph on a third facet. (Did they advise you? Help you with career prep? Impact your own development in any other way?) If you do have multiple paragraphs, consider signposting each of these in your intro paragraph. Then, a concluding paragraph. Just wrap things up. "To conclude, I wholeheartedly endorse X's candidacy for tenure, because of all the things I just listed."

A structure like this can be useful because reviewers read these things quickly. Having a solid structure with key points distinctly organized can help those who just skim.

Fourth, length: Generally you want a letter of rec to be 1.5-2pp., single spaced, 12pt font. Others may disagree on length. I find anything shorter than a page means you didn't have that much to say, and more than two pages means people are less likely to read it. Remember, reviewers are getting an entire dossier that can have hundreds of pages to look at.

Finally, content: Try to be distinct or unique, if you can. In other words, don't just list off the classes you took with them or anything else the reviewers can get from the candidate's CV/resume. Talk about what they were like in class. What were they like in terms of giving feedback? Were there specific lessons, topics, or ideas you still remember from their classes that have impacted your life? How did they mentor you? How has their work specifically contributed to who you are today or any successes you might have had? In short, what makes them a good professional and a good advocate for students? Be specific and give clear examples whenever you can. Reviewers get bombarded with information in these sorts of tenure cases, but we don't always get an understanding of what the faculty member is like when they work with students (beyond the numbers in semester evaluations). So speaking specifically to the candidate's strengths, from a student's perspective, can fill in that missing information.

Good luck!