What should be included in a support letter?
A great letter of support for a student will include the standard elements of a recommendation. 


Your relation to the student (lecturer, lab supervisor, academic mentor, etc.) 
The student's significant contributions to the school, the community, and the specific program.
The student's academic excellence
Other important factors. (Why are you being asked for this letter of support? What do you know about this student that supports their suitability for this award?) 


In all of these, be specific. Don't just say "X is an excellent student". Say "X is an excellent student, as evidenced by their insightful participation in class, their strong paper (won an award at [student conference in field]), and their continued dedication to learning. I was impressed most by x, y and z."