IHi Byudzai,
To each his own favourite method, but I've marked several Ab and E bass buttons as follows:
Have an assistant ( in my case, my wife) to hold the button in place using long nosed pliers (bind the action surfaces of the jaws with electrician's insulation tape or masking tape, so as not to injure the button or the bass board, first.
Mark the centre of the dimple to come with the sharp tip of a small knife.
Using a hand-drill ( sorry Paul) fitted with a new ( therefore sharp) 9mm (3/8ths inch) bit, while your assistant holds the button steady, gently but firmly and smoothly drill into the top of the button until only a slim, undrilled annular rim ( about 1 to 2 mm) is left untouched around the edge of the button.
The accordion can be laid on it's back on a soft surface (on a bed is good) in good lighting during the operation.
This slim rim is a great help in sensing the marked button by touch!
Wipe the sweat off your brow, and proceed to the next button.
This procedure does not involve removing any buttons from the instrument, and takes only about 10 minutes overall and gives a very tidy result ( for me, anyway).
Just make absolutely sure- before starting- exactly which buttons you want to mark, and centre the hole carefully: this is permanent!
And, yes, my wife and I are still on speaking terms!
This technique applies to older accordions, made in Italy, with solid bass buttons: it may not suit more recent accordions made in China :huh: