Soul music is a combination of blues, gospel, and the rhythm and blues. The artists of the popular Motown record label stand as examples to the finest in soul music; artists such as Smokey Robinson, Gladys Knight & the Pips, Marvin Gaye, Stevie Wonder, Diana Ross and the Supremes, the Jackson 5, the Temptations and the Four Tops. Writing soul music that stands up to these legends is a challenge, but if you're up to it, you might one day find yourself in good company.
Listen to the classic Motown artists. Listening to the music of classic Motown artists won't teach you how to write a soul song, but if writing soul is what you want to do, immersing yourself in the environment can help develop your ear for this type of music. The way a novelist should read books, a songwriter should listen to, and absorb, all the musical input he can.
Write lyrics. Jotting down a few random titles may help get your creative juices flowing. Think about the subject matter of many of the Motown hits. Love is a popular theme. Write down titles you can build the theme around. Start with a simple two verses, chorus, verse and chorus setup. Don't confine yourself to writing in that order. If you have the chorus first, write it down and work the verses from there.
Find the chords behind the lyrics with a piano or guitar to. Don't try to write the entire rhythmic structure of the song first. Get the sound of your song down. Soul songs work with a variety of chords, ranging from the simplest of major and minor chords, all the way up to extended chords. Experiment with different keys until you find what sounds good to you.
Record the chord changes while you sing your lyrics. Do your refining here. Even if you aren't a skilled singer, do your best to mimic the soul sound. Imagine your song in the hands of an experienced soul singer. If you're writing a song you hope to have another singer perform, you want to get the best representation of your song recorded so he or she has something to work with. If you can write music, put everything down on sheet music, or even a simple chord/lyric sheet.