STEAM GUARD
Sign in to Steam first. Redeem only when Steam asks for the verification code.
Sign in to the Steam account
Use the login and password received from the seller and check "Remember me".
Wait for Steam to ask for the verification code
Keep that screen open: do not close it!
Open the redeem page
Open the redeem page sent by the seller in another tab.
Enter or paste your single-use code
In "Your code", enter the 12 characters received with your purchase, with or without dashes. Then click "Redeem code". Use this code on the website; the Steam Guard code displayed next is the one you enter in Steam.
Copy the Steam Guard code and return to Steam
Paste the 5 characters into the Steam verification screen and confirm. Use the code as soon as it appears, within the indicated 5 minutes.
Unable to access? Contact the seller and request your code manually.