Go to the listing.
Click on Revise Listing.
Scroll down to the payment section.
Choose Flat Rate Shipping.
Choose a service.
Check off Free Shipping.
If you want to offer Free Shipping for US and overseas sales, do the same in those sections.
Note that you can offer Free Shipping to the USA, then choose another package of shipping destinations and enter a shipping fee there, but you still have to make it Flat Rate.