A cappella is a vocal song without instrumental music. There are many examples of a cappella songs, ranging from traditional barbershop quartet to street corner bebop and choral arrangements for four voices. A cappella songs put the emphasis on the vocal harmonies and use voice to its full potential as an instrument. Writing an a cappella song is a combination of writing a good lyric line and assigning voices to the lyric lines to showcase harmonic vocal structure in its best light.
Write lyrics to your a cappella song first. The lyric content is less a focus of the a cappella song than the way the words are arranged and harmonized. Typical lyrical content of an a cappella song depends upon the type of a cappella song. Lyrical subject matter ranges from spiritual topics to hanging out on the street and watching girls go by. A good example of a pop a cappella song is "The Longest Time" by Billy Joel.
Develop harmony and rhythm with the vocal parts. Decide if you want three-part or four-part harmony. With a three-part harmony, you will typically write a line for a bass voice, an alto voice and a high soprano. A four-part arrangement will include a tenor part. Use a digital recorder to work out the arrangements with actual singers if you can. If not, compose parts on the piano or an electronic keyboard.
Write a melody line for your alto voice. This will be the lead part. the alto singer will sing the lead vocal line throughout the song. Build other vocal parts around the main melody. If the lead singer is singing an E and the chord of the moment is a C, then you would write the fifth note of the C scale for the voice above the lead singer and the root of the chord for the bass singer. In the case of a C chord (built from the first, third and fifth notes of a C scale) you need the notes C-E-G. Because the lead singer has the E, write a C for the bass singer and a G for the soprano singer.
Write counter melodies for the bass singer to add movement to your a cappella songs. Because there are no instruments to give the song movement, you can write a bass vocal line that goes against the melody of the song. In this case, use a tenor or a second bass to handle the low notes of the harmony.
When possible, work with a group of singers. Writing an effective a cappella song is more about arranging voices. Sometimes it takes several combinations to find the harmony of a lyric as well as the counter melody that makes an a cappella song sound good. The more you can work with different voices, the better you can refine your song.