3DS flash carts:
Flash carts that are able to play 3DS roms/games on your 3DS.
DS flash carts:
Flash carts that play original DS roms/games on your 3DS and original DS.
Currently all 3DS flash carts also come with a DS flash cart because it is needed for the exploit to work.
First your going to have to download Gateway release 2.0B1NEVER, NEVER, NEVER!!! 2.0B2!. Go to the gateway website (I am not allowed to link it here directly) and click on downloads. When using google chrome right click the very first download link. **THE CURRENT VERSION ON THE GATEWAY SITE IS 2.0B2, DO NOT USE THIS! KEEP READING*** and select "copy link address". Now click the URL bar (the area where you type what website you want to go to) on that same google chrome window and erase all text currently in it. Now right click and paste the link you copied earlier, at the end of the text change "2.0b2.zip" to "2.0b1.zip" and press enter. The download should start.
Open the file you just downloaded and go into the "Blue Card (R4i)" folder and copy the "GW_INSTALLER.NDS" file onto your DS flash cart micro sd card. Start it like a normal DS game and follow the on-screen instructions turning off your 3DS when prompted.
Once that is done copy the Launcher.dat file inside the GW_BETA_RELEASE_2.0b1 folder onto your 3DS sd card (the large card inside your 3DS). make sure you have at-least 1 gb of space free on your SD card, Then put the SD card back into your 3DS. Turn on your 3DS and go into Settings -> Other Settings -> profile -> HOLD L + nintendo ds profile. The Gateway screen should pop up. Once again check at the top of the bottom screen and make sure it says 2.0B1NOT 2.0B2. Go over and select "Backup system nand" and ONLY "Backup system nand" never go into any other options!. follow the on-screen instructions. This will take about 10 minutes, just leave your 3DS alone during the process. at the end you will see "Done, press A to exit" press A then turn off your 3DS by holding the power button for 15 seconds. Take the 3DS SD card (the large one inside your 3DS) and plug it into your computer again. You should see a NAND.BIN file on the sd card copy this to your computer, burn it too a disk and upload it to dropbox. You NEVER want to loose this file. While the SD Card is still connected to your computer please delete the Launcher.dat file off it now.
the NAND.BIN file is now your nand backup.
nand:
This has your entire 3DS system on it, much like the hard-drive on your computer has windows on it.
nand Hardware MOD:This involves opening your 3DS and soldering small wires in specific places on the motherboard of your 3DS. This obviously voids your warranty, however this not only lets you fix your 3DS should it ever get the Blue Screen issue but you can upgrade and downgrade your 3DS whenever you want. (Update to play a retail cartridge like pokemon and get access to the ESHOP downgrade to play homebrew and use your 3DS flash cart). Attempt this mod only if you are comfortable with soldering! this is high risk and could very easily break your 3DS. This should be considered a last effort to fix your already Blue Screened 3DS.
3DS XL: HERE
3DS: HERE
Blue Screen:This is the screen that pops up when your 3DS system has been damaged or corrupted. This is the largest threat when using 3DS Flash carts. It is now fixable but only when you have a good nand backup the nand Hardware MOD and a Raspberry pi.
More info: HERE
This is how it looks:
Below is all known 3DS flash carts and the safest currently available firmware for that cart.
WARNING AVOID THE DIAGNOSTIC TEST OPTION ON 2.0B2