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

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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:



Thank you and much appreciated

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