Beyond just being a good writer, you'll need to have a good understanding of roleplay, persistent worlds, and the Ravenloft setting. Much of this knowledge you can increase through us but you must be willing to adjust to our needs and you must be willing to familiarize yourself with Ravenloft.
Along with the application should be send a sample of something you've written. This doesn't have to be related to Ravenloft in any way, though it may be an advantage if it's written for the fantasy genre.
-The length should be no more than 1000 words. A few lines can be sufficient though, if you feel it gives a fine example of your style and skill.