A Pokémon Gold, Silver, or Crystal Version cartridge.
An empty Box on the PC.
A Box that isn't full.
Pokémon to be cloned.
Another Pokémon to keep in the party.
1. Save the game.
2. Deposit the Pokémon to be duplicated into a Box on the PC.
3. Switch to an empty box on the PC.
4. You will be prompted to save the game for changing the box. Choose "YES" to the prompts, but shut off the power three seconds after the game starts to say "SAVING...DO NOT TURN OFF THE POWER.".
When the game is turned back on, there should be the Pokémon deposited into the PC in your party, and it should also be in the Box where it was deposited.
Another possibility is that there will be two of the Pokémon in the Box. [clarification needed]
Up to 5 Pokémon can be duplicated at a time under normal circumstances.
Items can be duplicated this way by making the cloned Pokémon hold the item to be cloned.
This can possibly damage the PC, and cause Pokémon to disappear. Details on this and why this can happen is not known.
Saving the game for changing the box is handled a little differently than saving the game normally. It consists of three steps:
The contents of the active box (before changing) is saved to the SRAM.
The number of the active box changes, and the contents of the new box is loaded from the SRAM.
The game is saved as normal.
If the game is reset after step 1 has finished but before step 3 starts, then the contents of the active box is updated, but the party (a part of the main game data) is not, resulting in Pokémon cloning.
Although the save file is protected by a 16-bit checksum and a backup copy, they only apply to the main game data, so this glitch is not prevented. Furthermore, this means that even if step 3 has started, as long as the game hasn't finished writing the main save file, the main save file would likely be detected as corrupt and the backup would be used instead, resulting in successful cloning anyway.
Overall, when using the change box method, the timing window to get a good clone is rather lenient:
On Crystal, the window is about 9–10 frames, beginning just a moment after the "SAVING... DON'T TURN OFF THE POWER." text is fully printed.
On Gold/Silver, the printing of the entire "SAVING... DON'T TURN OFF THE POWER." text, as well as a 16-frame delay afterwards, happens within the timing window, in addition to the aforementioned 9–10 frames.
Resetting a moment earlier than the beginning of this timing window may end up interrupting step 1 and result in the bad clone glitch, which is much harder to intentionally execute due to a smaller timing window.