Where do I scan the codes?

-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 (or scan it) and recive the the packs, etc. ingame.

-You can also redeem dorectly in the game app:

Login the TCG Live > select Shop button > select the Redeem tab > enter the code or scan it.