How to simply just click on size of a t-shirt and it adds it to cart

I want to create fast checkout experience for the customers and what i want to do is when they hover on a product it brings up the sizes and then they can simply just click on the size they want and it will add 1 quantity of that product with that size to the cart. Here is an example from Represent clothing.


Here is my site Read-Only: LINK
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