Whenever I write in a new fandom, I always fill one of my own prompts on an anon meme. It's a nice way to get the feel of the fandom and the characters, and because it's my own prompt, I don't feel as much pressure.

If you're really worried about being a non-native speaker, you could find a beta, but don't worry about it too much. A lot of my favorite fics are written by non-native speakers who have a better command of English than most native speakers.

So, go ahead and fill one of your own prompts. It'll probably relieve the stress of filling someone else's prompt.