Go to Setup->Shipping->Free Shipping - if you do not have the Free Shipping Shipping Method add it first by clicking on "View More Shipping Methods" and adding it.
Remember to select the Regions/States you want Free Shipping to work for! Leave the ones you do not want unselected. In the image below you can see Alaska, American Samoa, etc.. are not eligible for Free Shipping
Updated about 1 month ago