Avoid duplicating other flags, but use similarities to show connections…
This is perhaps the most difficult principle, but it is very important. Sometimes the good designs are already "taken". However, a flag’s symbols, colors, and shapes can recall other flags—a powerful way to show heritage, solidarity, or connectedness. This requires knowledge of other flags.
Often the best way to start the design process can be looking to one’s "roots" in flags—by country, tribe, or religion.