This tutorial will show you how to backup your DS cartridge save using a 3ds homebrew program called Checkpoint.
1. Insert your cartridge, and make sure the 3DS menu can see it. This is important because sometimes the system can't read the cartridge unless it's inserted just right.
2. Launch Checkpoint.
3. Use the DPad to select the game with the "DS" logo on it. Then press A to select the game.
4. Use the DPad to select the save slot you wish to use (the selected slot