Note: For the purposes of this guide, we'll assume you have either a 96- or 120- bass instrument (or close to that). These instructions may not necessarily be compatible with a smaller bass system).
After putting your accordion on, slip your left hand into the bellows strap and start feeling the buttons for one that has either a bump or a depression in it (it should be somewhere in the middle, closer to the inside of the instrument). This is your "C" pedal note. Press it. Then press the note diagonally-up from the pedal note. This is the "major" chord of the corresponding pedal note (C major). Now alternate pushing these two buttons while simultaneously pulling or pushing the bellows. You are now playing the stereotypical "oom-pah" accompaniment.
Note about fingerings: There are several schools of thought on what fingering to use for this oom-pah accompaniment. You can either use the fourth (ring) finger on the pedal note and the third/ second on the chord buttons, or alternatively use the middle finger for the pedal and the second for the chords. Both methods are widely used, although nowadays there is a growing preference for the former one. We suggest doing whatever you find most comfortable.
Now you know where two buttons are located. Using one of the fingerings described above, keep one finger on the pedal note but move the one on the chord button diagonally-up another notch. If you found the correct one, it should be "C minor". Again, alternate the pedal and chord for another, different-sounding accompaniment.
Repeat this process for the two buttons on the two outermost vertical rows to find the C dominant seventh and C diminished buttons. You should slowly start getting used to the fact that the C pedal and chord notes are on a sort of diagonal-horizontal row upwards, and adjust your hand position accordingly so that it becomes easy and natural to play all the buttons on this row.
Tip: For this, and future, lessons, don't shift the accordion down on your knees so you can see the basses. A professional accordionist never looks at their basses - and if the instrument is positioned correctly, it is impossible to see them anyway. If you can't play at all without looking, try sitting in front of a mirror and checking your left hand as a temporary aid.