Users can enter via this OneKontest landing page (which can be white-labeled, if you'd like) or by simply tweeting the contest hashtag (in this case #multikontest)
You can also require that the user be a follower of one or more of your Twitter account. We didn't require that for this sample contest.
There are several other options when you configure your own contest, feel free to set it up and contact us if you have any questions or need assistance!