This isn’t the same development that allowed pirates to load likely bootlegged ROMs from other regions on their 3DS — this new trick enables you to play a legit imported cartridge on your system. Again, this is not for piracy whatsoever and it does not play ROMs, just retail games.
Like most 3DS hacks lately, you need a 3DS with v4.5 firmware or older, so if you have updated your portable in the last nine months, you’re out of luck. By using a DS or 3DS flashcard compatible with the legacy firmware, you can run a special patched launcher file that modifies your system to load cartridges from other regions. You can even install more recent firmware afterward to play imported games that require it (with a few caveats for online play that might require game patches from region-specific eShops).