Check your email for the code (Please check your spam mail as well)
From the Nintendo Switch Console / Handheld
1. Select the "Nintendo eShop" menu on the Home Menu.
2. Select the account you want to use.
3. Select Enter Code on the left side of the screen.
4. Enter the code and click "Ok". (The code must be typed MANUAL, not copy paste)
5. Please take note, the following letters are never used o, i, and z.
Official source: Click here
From Nintendo 3Ds / 2Ds Handhelds and variants (New, XL)
1. Select the Nintendo eShop icon on the HOME menu.
2. Scroll left and select Add Funds.
3. Select Redeem a Nintendo eShop Card.
4. Enter the code and click "Ok". (The code must be typed MANUAL, not copy paste)
5. Please take note, the following letters are never used o, i, and z.
Official source: Click here
How To Use Nintendo Network Card From Nintendo WiiU Console
1. From the Wii U HOME Menu, select the Nintendo eShop icon.
2. Select Balance or press the + Button.
3. Select Nintendo eShop Card.
4. Enter the code and click "Ok". (The code must be typed MANUAL, not copy paste)
5. Please take note, the following letters are never used o, i, and z.
Official source: Click here