Step 1: Enable Action Replay
First, you need to ensure your DS or DS lite is compatible and fitted with an action replay device. If it's compatible and fitted with the action replay device, turn on your console with the game inserted and press the "R" button when prompted.
Step 2: Activate Code:
To activate the code that will make Mew appear automatically, follow these steps:
1. Press the "L" + "R" buttons to open the action replay menu.
2. Go to the "Codes" option.
3. Select "Manage codes".
4. Press "Add" to activate the Mew appearance code.
5. Input the Mew code: E494E1B0 1DAEE4C0
6. Press "Save."
7. Press "Exit" to return to the main game menu.
Step 3: Start a New Game
Now that you have activated the code, you must start a new game for it to take effect. When the game prompts you to select the type of Pokemon to start with, select any Pokemon. Mew will now appear in front of you.