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.
Close out of checkpoint, turn off your device, and insert your SD card into your computer
Go to sd:/3ds/Checkpoint/extdata and click on the Minecraft folder, then your backup
Copy the map you want to install to the minecraftWorlds folder
Eject your SD card from your computer and put it into your device
Section II - Importing the world
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.
If you did everything correctly, your world should now show up in the world selection screen, and properly load