Chaotic in conception but not in execution, Kelly's canvases are as neat as the

Chaotic in conception but not in execution, Kelly’s canvases are as neat as the proverbial pin

could you please describe the above statement for me.

This is the 3rd time I’ve tried to explain this having been logged out twice, so I’m going for the short version this time…

‘To be as neat as a pin’ is a simile which just means something is very neat.
‘Proverbial’ actually means ‘in the sense used in a proverb’ and even though this simile is not actually a proverb, the writer is using the same expression.

The author is saying that the artist, Kelly, may produce paintings which look chaotic, but in fact he has very deliberately worked neatly and carefully to achieve that apparent chaos… he works hard to make his painting appear chaotic.
So although his idea is to make something appear chaotic, that is not what actually happens - he executes the work (carries out the work) carefully, and not in a chaotic fashion.