for this challenge, i decided to learn the flags of the world.
i mainly used jetpunk, literally 100-200 times, to repeatedly test myself and learn through trial and error.
i also used anki web for spaced repetition, but the most important part was simply trying, getting flags wrong, and trying again.
i used different silly memory techniques and mnemonics to remember difficult flags.
for example, i used the grenade-like structure on the grenada flag as a visual mnemonic.
i also used categorization to group similar flags, like monaco, indonesia and poland.
these small memory tricks made it easier to distinguish flags that initially looked almost identical.