Start with a detailed outline. Know where your story is going and what key points you want to hit along the way. That gives you a framework to expand on.
Build complex characters. The more depth and development your characters have, the more storylines and conflicts can arise from their interactions and personal growth, making the story longer and more engaging. Also, introduce multiple subplots that intertwine with the main plot. Each subplot adds to the overall length and complexity of the story.
Describe the settings vividly. Spend time painting a clear picture of the places where the story unfolds. This not only adds to the word count but also makes the reader feel more immersed. And don't rush the pacing. Let events unfold naturally, allowing for more exploration and detail.