Log into your Seller account using your credentials. Once you are logged in, hover over the Orders tab and click Orders Listing.

Click three dots next to the order that you want to fulfil and then click View.

When viewing a single order, you will see all the details required to ship the product to the customer. 

At the bottom of the screen, click Accept Order.

Once you accept the order, you will see an option to Fulfil the order. Clicking it will open a popup, where you can add the tracking ID for the customer.