Go to the Contributions > All Contributions menu item and select the desired contribution by clicking on the supporter icon.
Please do whatever steps you need to fulfill the contribution - e.g., find the item they ordered, put it in a box with the address, arrange for the shipping, etc.
When you have completed contribution processing, click the Mark Fulfilled button in the top-right corner of the screen.
You also can bulk fulfill contributions. You can do this by selecting the contributions by clicking the checkbox next to them, then clicking the 'Bulk' dropdown in the top right, and then clicking Fulfilled.
Note: Marking a contribution as 'Filfulled' or 'Unfulfilled' does not affect whether the payment was processed.