To start placing your designs on products head on over to https://lockerstock.printexinc.com/ and sign in! I always recommend using the web interface for product building instead of the Shopify App directly.
1) From there select Your Products on the left hand side.
2) At the top choose Create New Product.
3) Choose the product you wish to place your design on.
4) Choose your colors and sizes you wish to offer the design on.
5) Give the product a name and description, this will appear on your store.
6) Using the mock up builder select Upload Artwork Image to upload the art of your choosing.
7) Save and finish!
Here is a video guide showing this process: https://www.useloom.com/share/c84c7a5dc4a64677867be078576f5e05
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