On your shop management page, you'll see the Listings option which pulls up and lists all of your items in your shop by title, price, etc, you'll see a little star to the far right hand side of each listing. Click on that little star and it becomes green, which means it's being featured on your shop page. You can have up to 4 items featured at a time, in order of selection (I think it's by selection), but you can technically choose more than that in case one of your features is sold.
Does that answer the question?