To feature one or several items, click on View/Manage Inventory in the left-hand column in your Dashboard. For each item you want to feature, check the box in its entry underneath the Edit Inventory box.
If you do not see the your item there, you can search for it by entering a query in the left-hand column and clicking Search and/or changing the parameters in the left-hand column and clicking on Refine Search.
Finally, fill in a Begin date for the Feature field in the Edit Inventory box, and click the Update Selected Items box. You will be asked to confirm whether to proceed with the item featuring at $2 per item.
Items stay featured for 30 days past the Begin date. There is no limit to the number of items that can be featured at once.