Begin the letter by typing the date. Skip a line and type "Dear Sir or Madam" followed by a colon.
Open the first paragraph with an explanation of how you know the student. Describe how you have seen the student's work evolve over the years or semesters. It isn't as necessary to identify the student as you normally might in an opening paragraph, because the letter is going inside his professional portfolio.
Discuss the student's relevant work, including portfolio pieces that demonstrate her mastery of a craft. Provide convincing reasons why someone might hire the student or let her into a graduate program. The pieces of work presented in the same portfolio are a good jumping-off point.
Offer an overall assessment of the student, and make a general recommendation. For example, "Bill is clearly a promising student who will be an asset to any marketing firm or graduate program. His work is professional and shows a keen understanding of his audience's needs."
Provide your telephone number and email address, in case a potential employer or committee member wishes to call you about the student.
Close the letter by typing "Sincerely," and then skip three lines and type your name. Print the letter on letterhead, and place your signature above the typed name. Keep a copy for your records.