To redeem a Twitch Gift Card, you need to:
Log into your Twitch account.
Visit https://www.twitch.tv/redeem.
Enter the gift card code in the available field.
Select your country of residence.
Agree to the Twitch Subscription Terms.
Click Redeem or Redeem Now to apply the balance to your Twitch account.
You can also redeem your gift card during checkout on any channel on twitch.tv