This tutorial will take you through importing worlds from the internet for Minecraft: New Nintendo 3DS Edition with Checkpoint.
This guide assumes you are on the latest 1.9 update.
What You Need
The latest release of Checkpoint (the .cia file, install with FBI)
You should already have this if you did Finalizing Setup when modding your console
A world of your choosing
Instructions
Section I - Prep Work
Open Checkpoint (the flag icon), then press (X) to see extdata
If you see 2 Minecraft icons, go to the one that says KTR-#-BD3#-00 on the bottom screen (the hashtags are the letters that it shows, it doesn't matter what they are)
Press (A) on it, then (A) on New... then (A) on Yes, name it whatever you want, wait for it to fully back up.
Press the (Home) button to suspend Checkpoint
Power off your console
Insert your SD card into your computer
Go to sd:/3ds/Checkpoint/extdata and click on the Minecraft folder, then your backup
Copy the world you want to install to the minecraftWorlds folder
Reinsert your SD card into your console
Section II - Importing the world
Power on your console
Open Checkpoint (the flag icon), then press (X) to see extdata
If you see 2 Minecraft icons, go to the one that says KTR-#-BD3#-00 on the bottom screen (the hashtags are the letters that it shows, it doesn't matter what they are)
Press (A) on it, then (A) on your backup then (A) on Yes to restore, wait for it to fully restore
Press the (Home) button to suspend Checkpoint
Open Minecraft: New Nintendo 3DS Edition, then press Play -> Load Existing World
If you did everything correctly, your world should now show up in the world selection screen, and properly load