First, the rules for writing any recommendations apply: the letter should be specific about your relationship to the student, and the length of time you have known them, and offer a candid and detailed assessment of their abilities. The most useful general rule I can give you to guide you as you write for the Marshall is that the program is both academic and ambassadorial, and the program takes both functions very seriously. We will be sending our students out as individual scholars and as cultural ambassadors who we believe will have a productive experience and be good representatives of Willamette, of their home communities, and of their country. This much is true of the Fulbright and most other programs that take our graduates abroad.
For the Marshall you must also stress the student’s potential to be an important contributor to their discipline, and the ways in which their service and leadership are not just strong, but extraordinary. Finally, one of the key ingredients for the UK programs is that the applicant be able to make a strong case for why the program they wish to pursue is the absolute best way for them to receive the training and experience they desire; any information you can add to support that case can be helpful (this curriculum would be difficult or impossible to achieve in the U.S. for example, or the UK program would allow them access to unique resources such as interdisciplinary or inter-institution collaborative projects).
Here are a few samples: