A Letter of Recommendation is a letter in which a professor/teacher/person of authority recommends the student to a particular course and college. It should focus on why you think the student is a good fit for the selected course and college supported by some examples for the same.
I can provide a few pointers to help you write a good Letter of Recommendation for your student.
Since you have not only taught the student but are also aware of her merits, your letter should shed light on her qualities that you believe make her the perfect candidate for the course. Explain why you think she has the caliber and what prompted you to write this letter (apart from the fact that she asked you to write it for her). You have also attended the college that he has applied to, so you know the kind of students the college accepts.
A letter of recommendation can come from any teacher or professor. You being a young graduate would not harm her chances of admission. The letter should focus on her qualities, any projects she undertook that brought out or showcased her caliber, and why she is a good fit for the program. For example, she may have been a Sports Captain that showcased her leadership and teamwork qualities. Or she could have taken part in debates or dance competitions. If she was an inquisitive student in class asking interesting questions and has insightful comments then mention that. Mention why she should be selected for the program, what makes her a worthy student, how she could excel in her chosen field, etc. You should focus on both, her academics and her personality. Ask your student about her goals and interests as well so that you can write in correspondence to that. Whatever you write should be supported with ample examples. And don’t go too heavy on adjectives. Try to sum up your student as a whole - successes, failures, personality, and academics.
Hope this helps you in writing a good letter of recommendation for your student. If you have any further doubts or queries, do mention them in this thread. I’ll be happy to help.
All the best.