First, just start writing down your ideas without worrying about structure. Let your thoughts flow freely.
To start a freestyle novel, begin with a concept or an image that really intrigues you. It could be a character, a place, or a situation. Then, start building around it, not being too concerned about making it fit into a particular genre or style at the very beginning. As you write more, you can start to shape it into a more coherent story if you like, but the key is to start with that initial inspiration and let your creativity run wild.
Begin by jotting down random snippets of stories, dialogues, or descriptions that come to your mind. You don't need to have a complete plan. Maybe you start with a single sentence like 'The old man sat on the bench, staring at nothing in particular.' And then you keep adding to it, exploring different directions the story could go in. Don't be afraid to make it messy at first. A freestyle novel is all about freedom in the early stages of writing.