How do you do it

You redeem codes, not cards. There are two ways to do it:

1) You go to the TCG Live web site:

https://tcg.pokemon.com/en-gb/tcgl/

In the main menu select "TCG Live", then select "Redeem Code". Make sure you have login in the site with your game account to be able to enter the code by typing it or scan de QRCode using the cam and recive the the packs, etc. ingame.

2) Or you can also redeem directly in the game app:

Login the TCG Live > select Shop button > select the Redeem tab > enter the code by typing it or scan the QRCode using the cam.