Pick the world or a continent, then name every country from its flag.
Name every country from its flag, for the whole world or one continent. Many flags differ only in the order of their stripes or the shade of a colour, so it's a good way to learn the near-twins: Chad and Romania, Indonesia and Monaco, Ireland and Côte d'Ivoire. If you've done the flags quiz on Sporcle or JetPunk, it's the same format. (Geographys isn't affiliated with Sporcle or JetPunk.)
Most national flags are built from a few designs: three stripes, a cross, a symbol in the top corner, or one emblem on a plain field. Shared colours often have a shared history. The red, yellow and green of many African flags come from Ethiopia's, long a symbol of African independence; the Nordic cross runs through the flags of Denmark, Norway, Sweden, Finland and Iceland; and red, white, black and green are the Pan-Arab colours of Jordan, Palestine, Sudan, Kuwait and the United Arab Emirates.
A few flags break the pattern. Nepal's is the only national flag that isn't a rectangle, Switzerland's and Vatican City's are square, and Paraguay's has a different emblem on each side. The flag images here, for all 197 countries, come from Flagpedia.