Answer:
You start by making up the sushi rice as normal. Once it is cooked, while it is still warm, you mix in a mixture of sugar and vinegar. The vinegar is made from rice wine. As the mixture is mixed together, traditionally a fan is used to cool the mix down.
Here's a great and easy to follow video I personally use, it shows you exactly how to make it!
http://www.howcast.com/videos/256-How-To-Make-Sushi-Rice