Lucky you :) Just click the REDEEM GIFT CARD link in the email. Create an account or log in and enter the code. You can always enter the code here. If you are having difficulty, log in first and then click the link via your email.
Gift cards work like store credit on hydrabotanica.com. Once you have applied the gift card to your account, it will automatically apply to your next purchase. If you do not have enough store credit in your account to complete the purchase, you can pay the remaining balance with your credit card.