This is your chance to toot your own horn. And you will need to do so. Many times, when a supervisor is too busy, they defer to the applicant to author the letter. You know you better than anyone so go ahead and scale back the humble. I imagine myself standing on an awards podium getting an award for "best nurse" for the unit. What would I hear, that is true, in the awards speech. Timeliness, time management skills, interdisciplinary team member, patient-focused...those are all good words. Look up your school's mission statement and try to incorporate some of those concepts if they are applicable to you. (If they are not, reconsider the school choice!) Also, you may consider the ethics of "waiving your right to view" this letter. Doing so strengthens the letter's impact, but can one really do so if one has authored the letter?