Double-click on the map as close as possible to each World country’s geographic center (centroid) that shows up on the right. The closer you click, the more points you get (max 100). Geographic centers do not include overseas territories, you are locating the centroid of the main land mass of the country. Centroids occasionally do not lie within the country itself or on land. You may have to zoom in to click an area next to an existing marker if the cursor is hovering by the existing marker.

Don’t want to do every country?
World 50 Country Centroid Locator