It is important to be sure that you are playing the chord on
It is very easy to drift onto the onbeat with these one per beat accents.
You could use a metronome to play the onbeats but I think it is important to feel the beat rather than just hear it - some part of your body should be moving to the main beat (foot tap or head nod for example)
The method outlined in the video works best for me - hitting the bottom note of a chord as the onbeat until you have the rhythm firmly established and then removing that actual hit and replacing it with an air shot.
Always start slowly until you are sure you have the feel of the rhythm and then gradually speed up.
This "skanking" style works for both traditional Ska and Ska-punk.
The tab is based on an A minor barre chord at the