Regardless of the genre, writing a music score takes the same types of skills. Such skills include focusing on the sort of music you want to write, instrumentation and effects, and idea development. As always, checking your score on the piano is helpful even if you have perfect pitch. This will allow you to hear more fully what it will sound like and may even generate more musical ideas. Please note that musical references here relate generally to classical music.
Knowing the Music You Want to Write
Know the sort of music that you want to create. This does not only mean understanding the genre, but also the different musical forms in that genre. A dance often written in the Baroque era, such as a Sarabande, was stylistically different than a dance popularly written for in the Romantic period such as the waltz. Understanding the type of music you writing is a very basic and important part of scoring, so make certain you have done adequate research if this is a new endeavor for you.
Research a musical form, if need be. If you are a little hazy on the type music you want to score, then consider listening to samples of the music. Download MP3 versions of the song or musical piece. If you need a more basic understanding, then consider buying a book related to the piece you are trying to create. You can find resources by ordering from sites such as Amazon.com, Hal Leonard Online and J.W. Pepper (see Resources below).
Practice playing music that you want to write. Manually playing out the music may help generate ideas for your own composition. It will also allow you to gain a better understanding of the music and how to put the score together.
Instrumentation and Effects
Understand instrumentation. Know the number and types of instruments needed, and if any substitutions can be made. If you are writing or transcribing a jazz piece and need three bass saxophone players, but only have two, consider using another wind instrument, like a bass clarinet. Experiment to see what works best to get the effect that you want.
Consider what sort of effect you want to create with the piece of music you are scoring. If a small group is playing, but you want them to sound larger, consider using loud dynamic markings like forte or fortissimo in the score. Additionally, you may add sections where instruments are playing essentially the same notes, to amplify the sound of the group. Also consider physically arranging the instruments so that their sound may be amplified or diminished according to your needs.
Use unusual instruments to create memorable effects. In the "Dance of the Sugar Plum Fairy" in The Nutcracker ballet, Peter I. Tchaikovsky uses a celeste to create an ethereal, dream-like musical mood. The celeste was a new keyboard instrument at the time that he wrote the piece, and to this day, no Christmas season arrives without that music being used in commercials and various other media. Utilizing a new instrument or new combinations of instruments further enhances the originality or your score and possibly the appeal as well.
Developing Your Ideas
Develop your ideas using a piano or small ensemble. This will allow you to possibly generate more ideas and bring other considerations to mind. It can also point out weaknesses or places of unintended dissonance that can later be edited.
Get advice from knowledgeable listeners. These include a composition or music theory professor, fellow composers or musicians, or listeners well-versed in the type of music that you are writing. Feedback from them can help you gain a better understanding of the genre you are writing for and areas of improvement.
Consider advice from others. Get advice from friends who do not listen to the sort of music you are writing. This can help to determine if the music is accessible to the average listener and whether it may have an appeal outside typical fans of the genre.
Get advice from the musicians playing the music. Though valuable, this may not be completely possible depending on the situation. If the piece being scored is a commission and only the band leader or music director is able to talk to you, then get a sense of how the musicians feel about the piece from this person. This will allow to make adjustments so that the music makes more sense and is easier and more enjoyable to play.