How do I set countries to have ideas? I can't seem to figure out what I need to put to assign the idea to a country.

Generally you would use "original_tag = TAG" (where TAG is the tag of the nation), however you may have some extra checks like if they're communist, or at peace, etc. depending on the idea and when it should (not) be available.

Where do we put the "original_tag = TAG"?

It should go in the idea's "available" box

Thank you.