How do I upload the designs in to my store?

If you are using a marketplace like Zazzle, Cafepress, or Sociaty6 you can upload the high res PNG's or SVG's directly and create products in the marketplace with that design.

If you have your own store on platforms like Shopify or WooCommerce and you are using companies like Printful or Printify for fullfilment you have to upload the designs in your account and you can create products and mockups from there.

If you do your own fulfillment you can use these online mockups generators.