This is a very difficult thing to do, that took me years to learn. The first thing to do is to copy the melody out and either the chord symbols or the harmony. If there’s a bass line, include that. You need to use harpistic figures instead of standard piano writing. The Salzedo-Lawrence Art of Modulating is a good guide to that and to improvising. It explains the difference in harp writing. You also have to be prepared to transpose, and sometimes create alternate harmonies. There is also a book by Verlye Mills on pop-music patterns for harp.