Before starting, go to the first PokeCenter (by the Pokémon School) and stand in front of the Delivery Man.
Step 1: Start at the final screen
Stand exactly as shown in the image below, or the timeline may be incorrect, causing you to miss your target frame.You should see the text: "You received xxx!"
Preview
Step 2: Set Up 3DSRNGTool
Input your game version and TSV in the upper right.
Input the initial seed in the upper right.
Check the Shiny Charm box if you have it.
Ensure you're on the "Event RNG" tab in 3DSRNGTool.
If you have the WonderCard file (.wc7) for the event, import it into 3DSRNGTool to update "Event Setting" automatically.
Alternatively, you can enter the data manually, but ensure it's correct for your event.
Note: Some events trigger a Pokedex registration animation, affecting RNG by changing the delay from 0 to 62 frames. Check 'No Dex Entry' in RNG Info to calibrate correctly. When the Pokedex entry screen comes up, try to close it quickly.