Write down your thoughts concerning an aspect of love, personal experience with love or a story about love. Don't filter yourself at first. Just write whatever comes to mind.
Review what you have written and choose a central theme. Eliminate lines or phrases that stray from the central idea.
Divide the song into verses and choruses. The verses should explain the idea or tell the story in greater detail, while the chorus should condense the idea of the ballad into a simple, repetitive refrain.
Organize your writing into rhythmic phrases. Many ballads rhyme, but this isn't a hard and fast rule. There should, however be a consistent rhythm scheme. An example of a rhythm scheme is to have the same number of syllables in your lines. For example, you might have 10 syllables in each line of your verses and 8 syllables in each line of your chorus.
Compose the music to your ballad. If you can play a piano or guitar, this will help tremendously. If not, you can hum or sing the song into a recorder and enlist the help of someone who can set it to music. Another option is to give the lyrics of the song to a musician and have her compose the music.