Well, you can start by choosing a perspective. Will it be from the point of view of one of the main characters from the original, or will it be a new character? Once you've decided that, start building the world. Describe the jungle, the strange creatures, and the atmosphere of Jumanji. Then, think about the conflict. It could be an internal conflict within a character, like fear of the game, or an external conflict like fighting against a powerful enemy in the game. From there, you can start writing the actual story, making sure to keep the action flowing and the dialogue interesting.