Well, what do you want the reader to know about your study skills and work ethic? That would seem to me to be the main question you have to ask yourself. So, put yourself in the shoes of whomever you expect to be reading your paper, and think about what they are looking for in an applicant. Then, think of all the ways you meet those criteria. Try to come up with specific examples and anecdotes from your personal experience that prove that you have all of the skills and qualities you have decided to talk about. Good luck.
If you have a hard time getting started, try freewriting. Set a timer for five, ten, or fifteen minutes then start writing about your study skills and work ethic, saying the things you would like your teacher or those who will receive the recommendation to know about you. Don't worry about grammar or punctuation and, no matter what you do, don't stop writing until the time is up. If you get stuck, just write "I'm stuck, I'm stuck" or "I don't know what to say" or "What should I say?" over and over until another idea comes to you. Then, when the time is up, look over what you've written and pull out the good ideas to use in your essay.