It's as easy as this:
Log in (with e-mail address and password).
Click on "My Account" in the log in menu.
Select "My promotions".
Enter the voucher code, accept the conditions and click on "Redeem voucher".
You're done; the amount will appear immediately in your betting account!
Important: Only one betting voucher can be active at a time. Each additional redeemed voucher will only be activated once the rollover conditions are completely fulfilled or the voucher funds are used up.