Barbershop music is special subcategory of ensemble singing. Although it can be sung by a large group, it also can be made very intimate, with only one person on each of the four parts. Arranging barbershop music requires a slightly different approach than arranging traditional choral music because the melodic line is displaced into the second highest voice rather than the top voice. With some practice and a decent knowledge of music theory, you can become proficient in barbershop arranging.
Draw a line on your staff paper which touches the left edge of the staves and which extends from the top of the very top staff to the bottom of the fourth staff.
Draw a bracket just to the left of your primary grouping lines that extends through the four-stave grouping.
Write in a clef for each staff. If you are arranging barbershop for women, the first tenor (top staff) will use a treble clef, as will the second tenor (lead). The baritone and bass parts will be written in the bass clef. If you are arranging for men, then write a treble clef for both the first and second tenor staves and a bass clef for the baritone and bass staves.
Write in the key signature on each staff to the right of each clef.
Write out the melody in the second tenor staff. In the rare instance you would like to move the melody from the lead, draw a dashed line between the staves to show where the melody leaves and returns.
Write in the bass part. (The bass part usually, but not always, has the root of the chord.)
Write in the first tenor part. If you are writing sixth, seventh or ninth chords, the first tenor usually has the upper pitch. If you are writing triads, the first tenor will sing either the fifth or the third.
Write in the baritone part, filling out the chords.
Check your music for any part doubling errors and problems with voice leading.
Write in your lyrics under each voice part.
Barbershop music is primarily homophonic (i.e., the harmony has the same rhythmic value). When you are writing, make sure all the parts and lyrics line up vertically.
If you must double on a minor triad, double the root and avoid doubling the third (you can double anything in a major triad). Eliminate the root from ninth chords, double the root of augmented chords, and avoid major sevenths if possible. (The diminished seventh is more "barbershop," and major sevenths lead up to a tonic resolution that isn't always desired.)