It's as easy as this:
Log in with your e-mail address and password.
Go to "My Account" in the account overview.
Select "My promotions".
Enter the voucher code under "Redeem code" and click on "Check".
You're done; the amount will appear immediately on your account!
Important: All vouchers have their own bonus terms, you should always check them for minimum odds or the validity period in your account under My promotions.