How to change the size from a ddlistbox to square boxes on the screen

I have not coded in 25 years, so I may have lots of followup questions.

I am trying to change the 'Select Size' dropdownlistbox on my product page to square boxes like these I have cut & pasted below.   I want the customer to select the size by clicking one of these boxes.  Could you share the code and tell me where I should add the code?

Thanks in advance for your help.

Hi,

This will require a 3rd party plugin.