First look around in #contest and see if there is some contest, that inspires you to enter. For example, when it is about a writing a story about a particular subject, you create a post using the mentioned (within the contest) keyword or category. Sometimes you can also leave a reply under the contest post.

That's it. With the keyword the contest creator can find all the posts that are entries to the particular contest.

Hope that helps ... If you have any questions, just ask <3