To redeem a Nintendo eShop gift card, you can follow these steps:

Turn on your Nintendo Switch and log in to your account.
Go to the Nintendo eShop.
Select the account you want to use the gift card with.
Select Redeem Code from the left-hand menu.
Enter the 16-digit code on the back of the gift card.
Select OK to confirm the code.
Once you have confirmed the code, the funds will be added to your Nintendo eShop account balance. You can then use this balance to purchase games and other content from the Nintendo eShop.