To upload customized products on Amazon FBM, you need to create a product page in the Seller Center first and fill in the necessary product information, such as title, description, picture, price, etc.

Then in the "Inventory" tab, select "Add a new SKU" and fill in the SKU information, including product name, brand, manufacturer, ASIN, etc., and then select the "Customize" option to fill in the relevant information of the customized product, such as color, size, material, etc.

Finally, save and submit the product, and it can be put online for sale after it is reviewed and approved. It should be noted that the production and delivery of customized products need to be completed by the seller, so the required raw materials and production equipment need to be prepared in advance to ensure that the order can be completed and shipped in time.