If you want to play old games, install retroarch cia version zip, extract the retroarch and cia archives from the zip and copy them to the root of your sd card, create a folder for roms in your retroarch folder and install retroarch cia through fbi. If you want to play new games, download cia from some 3ds game and install it in your 2ds through fbi. If you want to play both old and new games, do both procedures.