It's easier than a lot of people might tell you it is, the hardest part is actually sitting down to write the thing, whether you plan it out or not. What I'd suggest is keeping either a notebook and pencil with you, or using the notes app or some other one on your phone and just jot down whatever ideas or even scenes come to you. Then you can look through them and see if anything else comes to mind, but I'd stay away from questions like "how long will this be?", "will people like it?", "what if it's too silly/morbid/etc.?". Thinking like that will do nothing but stress you out, which is the last thing you want, believe me. Forcing yourself won't help at all, either, but that doesn't mean you can't just start and see where it goes. You'd be surprised how far you can get like that!
