Creating listings for the first time:
Once you have registered as a seller and enrolled in FBA New Selection, create your listings in Seller Central. Select the Fulfilment by Amazon
option when listing your products. We’ll guide you through the process of shipping your products to a fulfilment centre.Convert your existing listings to FBA:
Visit the Manage inventory
page in your Seller Account, choose the products you want Amazon to fulfil, and click Change to Fulfilled by Amazon under Actions.
Watch the tutorial here