Finger a chord on the frets. Any chord will do, but ska music tends to emphasize higher notes rather than lower notes, so consider chords that involve notes on the higher strings. Barre chords are often used with the highest three or four strings being emphasized.
Lift your fingers off the frets just slightly, so when the strings are strummed, you get a muffled "chick" sound. Strum a downstroke and get the quick "chick" sound.
Press your fingers down on the frets so that when strummed you get a clean sound. Strum a short upstroke. This will typically be played on the offbeat.
Alternate between downstrokes where the strings get the "chick" sound and then upstrokes where you get a clean, short sound.