How can I dynamically generate print-on-demand mockups from my custom app within my Shopify store page?

Hey guys. Essentially, I have an API that generates a custom image based on user input. I'm trying to figure out how I can accept that user input from my Shopify product store page and fetch the custom image from my API. I want that image to be reflected on a t-shirt as a mockup as well. Where do I start with doing this in Shopify?