Listen to ska music. The first step in writing any type of song is to listen to the genre and identify the qualities you like the most.
Mimic the ska rhythm on your piano or guitar. Ska is known for placing the emphasis on the off-beat, or the 1/8 beat. Counting 1, 2, 3, 4 is counting the 1/4 beat, and ska emphasizes the beats in between the 1/4 beat. Play along with a ska song to get the rhythm down perfectly.
Use major chords. Ska is a happy, upbeat genre, and major chords help create a lighthearted tone. Consult a songbook or online song library to learn popular ska chord progressions.
Write lyrics about love or politics. Because ska came out of Jamaica during a politically unstable time, many ska songs deal with politics. Even contemporary ska songs deal with politics. If politics isn't your thing, love is the other topic of choice in ska music.
Make the song danceable. "Skanking" is a dance inherently tied to ska music. The ska song you write should make people want to get up and hit the dance floor.
Add horn lines. Ska is notorious for its use of trumpets, trombones and saxophones. The guitar and keyboard usually keep the off-beat rhythm, while the horn section plays a melody or lead. You can write the horn line on your guitar or piano, then transpose it for the appropriate instrument later.
Record the song. Even great songwriters can forget what they had in mind. Never rely solely on memory when writing a ska song. Use a digital recorder, 4-track, cell phone---anything that will preserve your new creation.