Learning to read the bass clef of the accordion assumes at the very minimum that you can work out the notes of the treble clef from their positions. Ideally you can read the treble clef very well. The bass clef is linked to the treble clef (right hand clef) and can be worked out from it. Accordion music is based on standard music notation and is almost identical to piano music, with the notes F, A, C, E in the spaces and G, B, D, F in between the lines. You can remember these by the word "face" and by the sentence "good boys deserve fruit," commonly taught in music instruction.
Identify the first left hand (bass clef) note in the piece of music and name what the note would be on the right hand (treble clef), based on its position on the stave.
Count two tones (three notes) up from the note you named, counting the first note as the starting note, e.g. if the note is A, then count "A, B, C."
Play the C note with your left hand. You have played your first bass clef note. This is the two tone rule: between the treble clef and bass clef are two tones.
Go to your next bass clef note.
Count two tones, remembering to count the starting note as one, e.g. a D#.
Count two tones, "D, E, F," adding the sharp sign at the end to arrive at F#. You've worked out your second bass clef note, this time a sharp.
Practise this two tone (three-note) rule to systematically work out all your bass clef notes.
Pencil in the notes you work out as you go until you can fluently read the bass clef without working it out from the treble clef.
Repetition and regular practice, ideally at least twenty minutes a day, is the key to mastering a musical instrument and notation. Learn the right and left hand separately before attempting them together.
Make sure you count your starting note as one, e.g. if your note in the bass clef is an A in the treble clef, count the A as one, then B as two and C as three. It is three notes but two tone (A-B, B-C).