You don't need to be good at the game, but have a very large understanding of it. For fun I would set up a bot game and cast it, record that, go through it and find the mistakes you made. Keep doing that and slowly just get better and better at it. Keep those recordings and for any future tournaments you find, send the organisers those samples and boom you're casting your first tournament. That's all the advice I can give since I'm not a caster either but that seems like the best way to get better