Discuss the target audience for the music book with your client, or whomever you are arranging the music book for. The intended audience is crucial for your arrangements. An arrangement for the general public, for instance, will require you to condense the original scores into a piano/vocal arrangement complete with guitar chord diagrams. An arrangement for an orchestra will require the opposite. You will be required to expand the original sheet music to accommodate a full orchestra, which may entail writing additional parts.
Write down some ideas about the arrangement you want to make. Use music staff paper to jot down some sample arrangements. If you're arranging a simple piano/vocal sheet, determine where the melody of the song lies and transcribe the melody first. Once you've written a melody, you can begin adding bass and other rhythm elements to support the melody line. Piano/vocal arrangements should only contain the essence of the musical composition and should not be over arranged. If you're working on an arrangement for a choir, orchestra or a marching band, you will need to decide which instruments you'll have to write parts for and then determine how to arrange those instruments.
Choose a music notation program to help you arrange your music book. There are several music notation programs available, all of which work similarly. These programs range from expensive to free. Music notation programs allow you to drag and drop the elements of a music score, which makes arranging musical parts much quicker than by hand. These programs also allow you to preview your arrangements using real instrument sounds, which can help you determine whether or not your choices are correct.