If you already flashed ntrboot to your flashcart, you can follow Installing boot9strap (ntrboot) for instructions on how to use it.
Installing boot9strap with ntrboot requires a compatible DS / DSi flashcart to flash ntrboot to. The current list of compatible flashcarts is as followed:
There are three different methods of flashing ntrboot to your flashcart.
The usage of this exploit, regardless of flashing method, requires access to a small magnet if the target device is of a folding style (any 3DS family system that is not the old 2DS with a sleep switch). This is because the exploit requires your device to enter sleep mode while still having access to the buttons.
To test if a magnet will work, hold it on or around the (A)(B)(X)(Y) buttons while the device is powered on to see if it triggers sleep mode. If it does, both displays will go black as long as the magnet is held in that spot.
Note that the flashcart will not be able to be used for its standard functions while the ntrboot exploit is installed on it (except for in the case of the Acekard 2i, which remains functional on 3DS systems with custom firmware installed only). There are optional steps at the end of the ntrboot flashing instructions to remove it from your flashcart when you are done.
Note that in some rare circumstances, it may be possible for the flashing process to brick a counterfeit flashcart and render it permanently unusable. This is unlikely, but nevertheless only original listed flashcarts are supported. To reduce the chance of receiving a counterfeit card, it is recommended that you use a reputable site to buy your flashcart (such as NDS Card)
This method requires temporary access to a second 3DS family device that is already running some kind of custom firmware (such as boot9strap or arm9loaderhax).
This method requires temporary access to a Nintendo DS or Nintendo DS Lite that is compatible with your flashcart. Note that this method is not compatible with the Nintendo DSi.
This method, once ready, will require temporary access to a Powersaves. This method is not yet complete, and as such is just a placeholder page.