Request that the person you are recommending send you information including awards, major activities, academic honors, hobbies and any other information you might use in the reference letter. Ask the person to tell you why she would like to become a member of that particular sorority. Tell her to let you know of any unusual activities she has engaged in that would be appropriate for the sorority to know. Suggest that the candidate send you a picture of herself to be included with the letter. Sororities value having a picture so they can put the descriptions together with a face.
Tell in what capacity you know the candidate and for how long. Mention that you are qualified to write this letter of reference since you were a sorority member yourself, and indicate what chapter sorority you come from.
Mention the candidate's qualifications in terms of interests, hobbies or anything that makes this girl different from the other candidates. Examples would include if she is an artist whose work is displayed at the local library, if she won the national debate award, if she was her high school's first girl to win a tennis championship, or if she took a semester abroad to learn about the local culture and participated in a school assembly program sharing her experience.
Describe her personality and emphasize why she is fun and a pleasure to be with. Point out if she has an unusual sense of humor or can be the life of any party. Point out how she would fit in with the other sorority members. Add that you feel certain she will be capable of contributing to the organization.
Send the letter as soon as possible. Address it to the sorority c/o the university where the candidate is applying. Use "To Whom It May Concern" as your salutation.