Gather all the information you need about the musician. If you are writing about a living musician for a promotion, it will be easier because you can mainly just interview him. Remember that you must have a neutral perspective in this case, because it is for record executives, booking agents and media contacts to review. If you are writing an account of a dead musician, this can take months of research, reading and digging into archives. Reliable sources of information include magazine and newspaper archives, birth and death records, photographs, diaries and existing in-depth interviews with the musician. Check health records, public and private. If you are chronicling the progression of his music and how it evolved, it will also be important to analyze the composition of the music, or read the work of someone who specializes in this.
Interview as many people as possible who know and/or knew the musician: parents, siblings, wives, husbands, girlfriends, boyfriends, friends, band members, producers, record label executives, fans. It is best to understand the musician as a person, including what moved her emotionally and what her musical influences were. Use the tape recorder to make sure you do not miss anything. Make sure the interview subjects know you're writing a biography and what perspective you want to write from. Gather any written material or photographs interview subjects might let you use.
Understand the musician as a performer by watching videos of his performances and/or interviewing those who specialize in musical performance. How he acted as a performer greatly influenced his success as personal life as a musician, including how he prepared for a performance, how seriously he took it, and how far he would go to provide a memorable performance. Was money very important to him? Were the fans? Ask yourself questions that people who really want to understand the musician's mentality would ask.
Combine all of your information, photographs, interviews and personal perspective into the biography manuscript. Make sure the biography is in the third person.
Hire a fact-checker, or do it yourself if you're trained. If it is a biography for promotional purposes, let the musician look it over it.
Edit the biography yourself if you're trained, or hire an editor to do this.