This strategy is not actually a remarketing strategy. It is a really cool way to reach new people who are similar to an audience you care about.
You just need to choose a custom audience, a conversion tracking pixel or a page. Then choose a country to target. Optimize for similarity or reach, and finally click on create Audience.
You have to make sure there are at least a minimum of 100 people from the same country in the Source Audience you are using to create your lookalike audience.
Marketing is all about statistics. There is a huge possibility that if you find a group of people who have identical needs and interests to another group of people you are working with and giving you great results, it’s possible that you can get the same positive results from that new group of people.