Ska rhythm is basically playing the chords on the upbeat. (not exactly but I'll use that to explain). Ok say you count 1 & 2 & and repeat. You play a muted dowstroke on 1 and 2, and on the &'s you'll up-strum the chord. So it would look like this.


1 & 2 &
--x--6--x--6--
--x--6--x--6--
--x--7--x--7--
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It's alittle tough at first, but just think of playing a straight rhythm. Your muting the strings when you play a downstroke, and your playing the chord on the way up. Start slow and you'll soon get it.