In my experience, the simplest way is just using a reliable GPT (OpenAI, Claude, Bing AI, Gemini AI...) and your prompts. Start with a title, a pre-defined plot, and instructions. Make the GPT create a starting point for the story, name it chapter 1 and move along as the story evolves with your characters, events, etc. It involves the AI and your imagination to transform and evolve the story . The key is the way you manipulate the prompts in such a way that the AI remembers your past discussions. If not, restart again to where the storyline stopped that you accepted as correct, then create a new session. It take practice to make it right. Gemini AI can do it in one sitting, or even mixing the free accounts (for mixed responses) from OpenAI and Microsoft Copilot. If it is a novel, then you need to pay for additional requests or install open source LLM for unlimited use. There are a lot of free and paid options if you search the net, so your opportunities are endless.