Nomination letters and recommendation letters have a lot in common. In both cases, you are writing a letter on behalf of a candidate in order to convince a third party that said candidate is worthy of a title, job, award, or scholarship.
With both letter formats, it's your duty to sell the candidate. You have to talk about their strengths, mention specific anecdotes that speak to their character, and relate your experiences observing or working with them to convince a panel or judge that they're the most qualified.
The big difference between nomination and recommendation letters is that recommendation letters are part of a larger application; nomination letters stand on their own. You have gone out of your way to endorse a candidate, but that candidate doesn't have the chance to speak to their own talents and achievements. You have to do so for them.
When you write a nomination letter, make sure you know what the requirements are for the title, position, or award. Once you know, address each of those requirements in the letter and be sure to describe how (and it what unique ways) the candidate meets those requirements. You are not just filling in the cracks here. Your letter has to function as the resume and bring up the big, obvious jobs, awards, grants, and community service the candidate would normally highlight themselves.
Index of letter of recommendation templates