Step 1: Opening Paragraph
Opening should indicate general but not excessive warmth for example “I am writing this letter on behalf of XYZ” and follow is by some explanation about who you are and how you know the prospective student for whom the letter is being written, for how long do you know the student and in what capacity. Then go on to compare the student in terms of discipline and topical/thematic focus. For example, “XYZ was among the top young phycologists in our department and shows a great deal of promise.”
Sufficient description of the research work should be provided, dissertation/thesis can also be used. The most important things a letter does is it provides a context on candidate’s research in awider scope.