Before starting the system transfer, ensure that you back up your current 3DS SD card on your PC. Charging your 3DS systems during the transfer isn't required, but it is recommended that both units are already fully charged and that you have the charger on hand.
Cancelling the transfer when it has already begun could result in loss of data, so ensure you plan the system transfer when you have the time. The process can take up to 30 minutes depending on how much data you have stored on your old device.
The System Transfer process
1. On the HOME Screen, select System Settings Icon and tap open.
2. Select "Other Settings"
3. Tap "3" and then "System Transfer"
4. Once the Internet connection has been established with the Nintendo Server tap "Transfer between Nintendo 3DS Systems" Menu Option.
5. On the 3DS XL, select to "Receive Data" and on your old 3DS, select to "Send Data".
Follow the onscreen instructions to perform the Transfer, Little Pikmin characters will move between the 3DS Units moving your Data. Stay with the consoles throughout 6. the transfer to confirm necessary actions on screen.
7. When the transfer is complete you be shown a screen showing "Number of remaining Transfers" and "Days until next Transfer".
The new 3DS XL will contain all of your data and be ready to use, the old 3DS will format itself, erasing all data and sensitive information as part of the transfer.
Alternatively, you can also watch this handy video: