Get the basic rhythmic elements of ska in your head. The best way to do this, of course, it to listen to as much ska music as possible, paying special attention to "backbone" sounds of the genre: a fast pulse in 4/4 time in (usually) major keys with accents on the backbeats (beats 2 and 4) and instrumental punches on the offbeats. Keep this basic collage of sound in mind as you start to compose other musical elements to go with it.
Compose vocal parts. Ska vocal lines are generally fast-moving and punchy, but they stay in a fairly narrow, mid-low vocal range. Vocals may be performed by a single singer or a group, but group singing will tend to have limited harmony: lots of unison and only splitting into occasional, usually two-part harmony that heavily emphasizes major thirds. The song form should be standard verse-chorus with optional bridge and square, repetitive phrases.
Write text. The lyrics for your vocal line can be about pretty much any subject matter you chose, but in true ska style they will usually have a lot of short syllables rather than long vocalize-style singing.
Fill in the rhythm section parts. This will tend to be along traditional lines as outlined in Step 1 with fast, backbeat-heavy drums, a quick walking bass line and guitar, piano and organ playing quick chords on the offbeats.
Write horn parts. Like the vocal harmonies, the horn section features (usually consisting of trumpet, sax and trombone) will tend toward lots of unison and octave playing with some simple two-part harmony. Horn soli sections should start and end the piece and be interspersed between the verses and as fills during empty sections of the singing. The horns should also join the rhythm section in providing background offbeat chords.
Make room for any solo breaks. Your song doesn't necessarily need improvised solos, but it's common to include them. Choose any melodic instrument you want to feature (bass and drum solos in ska are more rare), but bear in mind that trombone solos are somewhat more popular than solos for other instruments in this genre.