Once you’ve signed up and logged in, click on the blue “Upload Design” button in the upper right corner of the homepage. Then, just click and drag your PNG file to the center, or click on the center to browse your files for your design. Please make sure your art file is as high-resolution as possible!
Specific requirements can be found in the Uploader, and in the handy design guide found here. If a design isn’t within these image requirements, then the fulfillment center won’t be able to print it.