According to the Shopify help section, to activate a shipping zone, you should follow these steps:
enter Shopify admin;
go to Settings > Shipping and delivery;
next to the shipping profile where you want to add a shipping zone, click Manage rates;
next to the group of locations where you want to add a shipping zone, click Create shipping zone;
enter a name for the shipping zone; select the countries and regions that you want to be in the zone;
click Done;
add any rates that you want for the zone;
click Save.
To use a buy button you should create it in your Shopify account. Then you will need to copy the automatically generated code and add the code to the webpage or blog post where you want the button to appear.