Register.
Answer some simple multiple choice questions (each have a clue).
You need to get the questions right, but they let you change them if they’re wrong... very easy!

"You can repeat the quiz as many times as you like and you will get different questions each time. You can only win a maximum of one free product per household every 3 months. However, for every 3 friends you have referred that play the quiz, you will be able to play and win again immediately. No cash alternative available. Open to residents in US, Canada, UK, Germany, Switzerland, Austria, Denmark, Netherlands, Belgium and Luxembourg. Nikwax, or its representatives, reserves the right to substitute an equivalent prize should the prize stated be unavailable. "

http://www.nikwax.com/en-gb/webquiz/index.php