Well, start with a strong opening that showcases the power and danger of poison jutsu. Maybe have a scene where a victim is slowly being poisoned and the effects are described vividly. Then, develop your characters. If you're writing about Naruto using poison jutsu, think about how it changes his personality. He might be more cautious, knowing the lethality of his new power. For the plot, consider adding some twists. For example, the antidote to a powerful poison jutsu is something very unexpected, like a common flower found only in a certain part of the village.
To write a good Naruto poison jutsu fanfic, you should focus on the details. Describe how the poison jutsu is performed. Is it a cloud of purple gas? Or maybe it's a liquid that seeps into the opponent's skin? Also, create conflict. The character using poison jutsu could be at odds with the traditional ninja values of the village. They might be seen as an outcast because poison is not a typical 'honorable' jutsu. You can also include some action - packed scenes. For instance, a battle between two poison jutsu users where they try to outwit each other with different types of poisons and antidotes.