The best way to do it is to be completely honest. Through your writing, the reviewer will see what type of person you are. The best way to explain it would be to write a letter similar to a Graduate Application Essay. Search online to find how to do this because it is too lengthy to explain. Take quite a few occasions to reread your letter to make sure it says everything you want, the way you want. The Honor's Society is not just about GPA, it is about well-rounded individuals.