Listen to the artist's previous recordings. Take note of recurring song content, pacing and style.
Research the artist's listener demographic. Consider the expectations the audience has of the artist. For instance, does the audience appreciate the artist's older work or newer work?
Consult the artist's image for determining what is in character with the artist. For instance, you do not want to write a song about politics for an artist known for his dance records.
Sit down with the artist and take into account her motivations. What is she hoping to achieve in tone, content, style with the song?
Pitch an idea of the song to the artist and see if he goes for it. If not, work with him until you find a middle ground from which to collaborate.
Write the song and record a reference track for the artist to listen along with. Lyrics are not enough to sell a song, so the artist and label will need to hear how it sounds with music and vocal delivery.