First, you create a product listing in your Seller Central account. After you create your listing, Amazon assigns your product an FNSKU. You send this FNSKU number to your supplier in China to be printed on the box. So Amazon knows this product belongs to your account/yours.
Next, make a shipping plan. During this process, you tell Amazon that you want to send them the product, along with details about your shipment, weight, volume, dimensions, etc. At the end of this process, Amazon will provide you with a “shipping label.” You also give these labels to your supplier, but this time print/label them on the outside of the master carton.
Also, you need to have a China Freight Forwarder with the carrier to handle the shipment from the port to the Amazon warehouse. You can search for US carriers, there are many