Hello,
You may have seen that there's new Custom Maps available in Explore Analytics!
In order to leverage it, you need your custom map to be in a GeoJSON format. In this thread, please feel free to contribute any resources to help get custom maps in the format you need!
MapShaper - One of the most common things you'll come across is that your map will be in a different file-format.
MapShaper allows you to upload a lot of different filetypes, and re-download it as a GeoJSON file (which is what you'll need for Explore Analytics).
MapShaper also has a really cool 'simplify' feature that lets you streamline the shapes a little -- if you don't need every nook and cranny of the coastline, simplifying gets you a much lighter, and quicker to load, file!
This great GeoJSON tool allows you to select specific countries (or continents) to get a custom GeoJSON file: