Put the 3DS' SD card into your computer, then copy every file from it (even, if there are, hidden files) and paste them in the New 3DS XL's micro SD card.
Or you can also use the "Data transfer" option in the settings of your New 3DS XL.
How do I transfer data from 3DS to New 3DS XL, especially with the two systems using different sized SD cards and the New one having only 4 gigabyte capacity while the old one contains 5 gigabytes? (preferably with step-by-step instructions so to lessen the chances of my screwing this up)