To start writing manhwa fanfiction, begin by choosing your favorite manhwa. Dive deep into its lore and characters. For example, if you like 'Hardcore Leveling Warrior', study how the leveling system works and the personalities of the characters. After that, think about a unique angle for your story. Maybe a new villain appears or a love interest for the main character that wasn't in the original. Then just start writing, don't worry too much about grammar at first, just get your ideas down.
Well, the first step in writing manhwa fanfiction is to immerse yourself in the manhwa universe. Let's say you're into 'Bastard'. Know the dark and gritty atmosphere, the complex characters like Jin Seon. Then, brainstorm ideas. You could take an event from the manhwa and expand on it. For instance, if there was a fight scene, you could write from the perspective of a bystander. Then, create a structure for your story, with a beginning, middle, and end, and start filling in the details as you write.