I'm not an expert or an Admin, but here's what I have worked by for the last year here and nobody's kicked me out yet.
1 - Creepypastas are a variety of short story, so you need to know all the fundamentals of writing any short story. Good English, all the rules and conventions of composing a story, use of imagery, description, and emotion, etc.
2 - A Creepypasta isn't a gore fest. You can toss buckets of blood around and it's going to just bore people. You can use gore, but it is only one possible part of horror, not the definition of it.
3 - Know the rules for this site. Some common first story topics are forbidden here.
4 - To my mind, a Creepypasta has the notion of something out of the unusual that becomes creepy and then it becomes creepier and then it moves into a horror.
5 - Read Creepypastas or listen to narrations of them. (or both). Get to know what is good and what works for you.
6 - Have the idea. My first four were based on my own real-life experiences. I've gotten inspiration for others from a wide variety of places. Don't use someone else's idea. Don't use someone else's characters or make a "fanfiction."
7 - Write up a solid draft. Spell check, grammar check and read it out loud. Do your best with it.
8 - Post it in the Writer's Workshop and work with the people that are there.
Lost episodes aren't accepted on this wiki without a spin appeal, however there are other wikis that do accept them which I'll be happy to name if you want.
It is generally accepted wisdom that it takes an experienced writer to pull off a lost episode story, but don't let that stop you if you really want to. Even if it isn't good, even if you never publish it, if that's where you want to start then I say go for it.
Alternatively, you can askew the normal lost episode format and instead of writing about an existing TV show, you can write a creepypasta based on one you made up. A lot of successful television-based pastas are based on fictional shows. In addition, such stories typically don't require a spin appeal, though if your story follows the "lost episode" formula too closely it may still require an appeal.
You mean like one of those character sheets for roll playing games? You can post it, but every time I have ever seen one posted the Admins have deleted it very quickly. Often it's gone within minutes. Such a character sheet is not a story.