All we know that user needs to have a 3ds flashcart or geniune cartridge to play game on 3DS. As one geniune cartridge contains one 3ds game which means you can only play one 3ds game with one cartridge. While one 3ds flashcart is necessary and it can support unlimited 3ds game with one cart. If you do not have or can not afford a gateway 3ds or sky3ds+, you can install FBI to install and play CIA games.
Beffore installed FBI, you must have already have a CFW installed: such as ninjhax, freakyhax…
1. Download FBI.zip and extract the folder. Copy the FBI folder to the 3ds folder on your 3DS SD card.
2. Copy the fbi.cia file in extracted folder to the root of your 3DS SD card.
3. Insert your SD card into your 3DS and turn on your 3DS. Access to homebrew launcher and there will be FBI icon. Run FBI and follow the screen instruction to install it.
4. After installed, there will be an FBI app in 3DS home menu. So you don’t have to use homebrew launcher every time you wanna install something new
5. If you want to install CIA files or games, just copy .cia file to the root of your 3DS SD card.
Note: it will spend some time to install large .cia files. It is normal to take 20-30 monutes to install it.
Source from wololo.net