If you play what is often called a Second Accordion in, say, Scottish Dance music, your job is to provide chordal accompaniment to enhance the music. Generally, this means playing a short block chord on the off-beat of the music but can also involve (with care) block chords on the on-beat. The idea is to add lift to the music so the off-beat chords are meant to be very short - almost percussive.
The way I leaned to do this was to start by only playing music using the Three-Chord-Trick (tonic, dominant seventh and sub-dominant chords). The first step is to minimise having to move your fingers much between chords - so you need to use suitable inversions of the chords. This doesn't matter very much with an accordion as the fact that you may be playing notes that are an octave apart anyway (if you use the appropriate coupler) makes inversions sound correct.
The stave below suggests some finger patterns in the key of G major.
Notice how little the fingers need to move from one chord to the next. In general, you should only need to move two of the three fingers for each change, leaving the third finger to act as an anchor. The finger numbers are only a suggestion, you could use different fingers - the key is to keep one as an anchor as you change.
Next, follow along on a piece of music, playing the chord percussively on the right hand in time with the chord in the left hand.
As your skill develops, you can add other chords while still minimising how much you move your fingers. Some suggestions are given below.
Once you get the rudiments in hand, you can go on to some more complex chords and, if necessary, move your hand completely to change the tone of the chords as you play.
Beyond that you could try to develop rhythm from your right hand. More ››
A particular example of chordal accompaniment is the Second Accordion in a Scottish Dance Band. More ››