When writing an Iron Fist OC fanfiction, begin by fleshing out your OC. Develop their physical appearance, skills, and weaknesses. You can give them a special weapon or ability that sets them apart. For the story, think about how your OC will enter the Iron Fist world. They could be a student at a martial arts school where Danny Rand also trains. In terms of plot, you might create a conflict that involves a new villain who is after the power of the Iron Fist and your OC gets caught in the middle. Write dialogue that reflects the character's personality. If your OC is sarcastic, let that come through in their conversations. Also, don't forget to add elements of the Iron Fist mythology, such as the importance of chi and the history of K'un - Lun.
To write an Iron Fist OC fanfiction, start with your character. Consider their personality traits. Are they brave and headstrong like Danny, or more cautious? Then, think about their origin. Maybe they're from a different part of the world with a distinct martial arts style. For the plot, you could have the OC stumble upon the power of the Iron Fist in an unexpected way. For example, they could find a hidden cave in the mountains that has a connection to the Iron Fist's power. As for the writing style, use vivid descriptions to bring the martial arts battles to life. Describe the punches, kicks, and the flow of energy. Also, make sure to research the Iron Fist lore so that your story fits within the existing universe.