Pick your style. Even though Broadway songs are defined by a specific sound, there are now several different genres within the musical style. This includes everything from opera to rock to jazz and more traditional sounds. You will want to pick the style that best fits with the characters and play that you are working with.
Set the scene. Because Broadway music has to fit with a plot and characters, you will want to make sure that your style and sounds fit with what is happening on the stage. For example, if a character has just lost his loved one, make sure that you express this with the instruments, pace of the music and wording that you use. Getting a general idea for the sound you want will help you to lay out the details later.
Write your lyrics. Once you know your character and mood, you can set the lyrics. You will want to make sure that the lyrics either describe the mood that is taking place with a certain character or scene or describe something that is happening. This will help to pause the Broadway play and push it forward with the concepts at the same time.
Set the instrumentation. Typically, Broadway music will have an orchestra pit underneath it, so take advantage of the arrangements that you want, ranging from violin sections to woodwinds, brass and piano, as well as percussion. Knowing what you want in your song first will help you to define it more clearly.
Put in the melody. You will want to move back to your lyrics and define the melody that is going to be used with the character. With this, make sure that you have the right range and voice for the character. For instance, if a female is singing, set the range to be at a middle C to a high E for an alto and a high A for a soprano. Knowing these different options will help you to set together the ranges correctly.
Let your instruments support the character. After the melody is set, you will want to continue by setting the harmonies with the melody. This can be done by finding the right key signature of the melody and then adding in harmonies to this. Make sure that you have a rhythm that is set, as well as a bass part that will hold everything together. The most important part of the instruments is to make sure that they color and define the singing that is taking place.