Start with the essential formalities. Write "Dear Student's Name." Begin with a thank you for the willingness to volunteer. For example, "Thank you for taking time out of your studies to volunteer your services to senior citizens in the community."
Use concrete terms. In the second paragraph, talk about the exact location, hours and tasks of the volunteer work. For example, "As per our discussion, you will be volunteering on Saturdays during March, April and May at the YMCA located on 1234 Main Street. Your duties include serving meals, raking leaves and cleaning dishes. Please report every Saturday at 8 am to this location. If you cannot make it on a particular week, please call (123) 456-7890 at least 1 week in advance so the departmental staff can attempt to find a replacement."
Finish the letter. Ask the student to contact your department in case he/she has questions or concerns. You can thank them again at this point. Add "Sincerely," followed by your name. Be sure to sign the letter after you print it.