How do I add a modal window pop up in my product page, using HTML coding?

[Navz]

I would like to add a sizing chart link and corresponding modal window (400 x 800 px) pop up for a sizing chart on my product page. 

The sizing chart is a PNG image that is already within the Shopify assets.

I would like to know what is the right code and where would i place each of the code in shopify.

For example, I am looking for something like this:

Modal Link - "Size Guide"

and the modal window:

Location of example: https://www.rmwilliams.com.au/comfort-craftsman-boot-chocolate-kangaroo-leather.html?lang=en_AU

Thank you and much appreciated