Every singer in a quartet sings a different part of a song. The lowest voice in a male quartet sings the bass line. The bass notes establish the chords of the song, which together form its chord progressions. The highest or second-highest voice usually sings the melody, or the recognizable musical line of a song. The other two parts fill in the chords, creating a full four-part harmony. A quartet song will usually start and end on the tonic or "do" of the key, the same note as the key in which the song is written, to make for easier arranging and musical completeness.
Some songs lend themselves more easily to quartet arrangements than others. The Barbershop Harmony Society identifies some criteria for a good quartet song. The song should have an easy-to-sing melody within a reasonable vocal range; a symmetrical melody supported by a distinctive, logical chord progression; and uncomplicated lyrics, driven by some kind of emotion. To determine if a song fits these standards, listen to it a few times through. If you can identify a pattern of chords, hear a distinguishable bass line and sing back the melody without too much trouble, it's probably a good fit for a quartet. It's especially important that an arrangement for a men's quartet has a melody within the singer's range.
An a capella arrangement includes not only the melody, but the accompanying backup chords that combine to produce a full, harmonic sound. Start by charting out the melody of the song to help keep your place as you arrange. If you can notate by ear, listen to the song and write what you hear. Otherwise, you can use pre-written sheet music of the song as a model and simply transcribe the melody line. Next, fill in the bass line. The bass notes should align with the song's chord progression, which you can also find from a pre-written chord chart or a piece of sheet music. Finally, fill in two harmony parts. Play your arrangement on an instrument or notation software when you're done to make sure everything sounds right.
There are a few ways to add lyrics to your song. The voice part singing the melody line should always sing the original lyrics, but there's a little more freedom with the backup parts. A simple option is to have the backup parts sing the original lyrics. Another technique is to have backup parts sing on syllables, such as "doo" or "bum," that align with their rhythms. If the song is slower, it may be more beneficial for backup parts to sing their notes on a simple vowel, such as "ah" or "oo," which will blend better with other parts and give the song a choral sound. You can also use elements of each of these techniques; for example, backup parts can sing primarily on syllables, but switch to lyrics for emphasis during certain moments of the song.
The best arrangements stand out by using unusual melodic lines, rhythms or lyrics. Spice up your arrangement by looking for spots to stray from a simple chord progression. Alternate using straight rhythms with syncopated ones, or give every part a different rhythmic pattern. Write in musical suspensions and resolutions or dissonance between two notes that resolves into a harmonic interval. Write in dynamic markings, denoting where singers should sing louder or softer. If it's a humorous or satirical song, consider changing some of the backup lyrics to reflect the tone of the song. Listen to the original song again for pickup notes you may not have originally written into your arrangement, or search for other arrangements of the song for ideas.
The first time a quartet sings through your arrangement, the singers will probably have thoughts about how to tweak it to make it more comfortable to sing. Listen to these suggestions and work them into your arrangement. Sometimes it's not obvious from a written arrangement that a certain interval is hard to sing or a musical line is too difficult. An arrangement is never completely finalized, as every quartet that sings it will have a different interpretation of the song. Let the singers have some say.