Redeeming the download codes on your 3DS is the most exciting and fun part. It is really easier than expected. After you had successfully get the download code for any games you want, just follow these instructions and you will be downloading it in no time.
STEP 1: From the HOME Menu, select the Nintendo eShop icon
STEP 2: Select “Menu” in top left
STEP 3: Select “Settings/Other”
STEP 4: Tap the box that says, “Redeem Download Code”
STEP 5: Enter the Download Code & get gaming!