Add Item to Checkout Process

I’m working on a site for sublimation printing where you choose your item, say a coffee mug, purchase it and upload a photo to go with the order so the site owner can print your photo on the item you order. What I like to do is add a shortcode or something into the checkout process, maybe in the “Step 3 – Payment Process” box or create a new “Step 3 – Upload Artwork” and change the current Step 3 to Step 4. I’m trying to figure out where to edit this at do so.