Instructions
Purchase a third-party Nintendo DS flashcard such as the DSTTi, Action Replay DSi or R4i SHDC.
Insert your flashcard into your computer's card slot. Most flashcards use a MicroSD interface, which comes standard on most modern notebook or laptop computers. If your computer is not compatible with your flashcard, you may need to purchase an additional adapter. Refer to your flashcard's documentation for precise information about the card's interface.
Copy your cheat database file from your computer to your flashcard. Examples of cheat files include "cheat.dat" for the R4i or "usrcheat.dat" for the DSTTi. If your cheat file is not compatible with the hardware you purchased, convert it with a cheat code editor or download a new cheat file that is already in the proper format. Consult your hardware's documentation for specific instructions if this the case.
Plug your flashcard into the Nintendo DSi, and turn the system on. You can now use your flashcard's interface to enable or disable the cheats you want from the cheat file.