Begin with a solid understanding of the anime's lore and characters. This is crucial as it will help you blend the gamer elements seamlessly. For instance, if you're writing for an anime like 'Fullmetal Alchemist', know the rules of alchemy and the world's politics. Then, create a gamer system that complements this world. Maybe the character can use alchemy in a new and godly way because of the gamer system. Develop your plot around how this new power affects the character's journey and relationships in the anime world. Write detailed descriptions of the character's actions and how their new abilities change the course of events in the story.
First, you need to choose an anime that you're really familiar with. If you love 'Attack on Titan', for example, then that could be your base. Next, think about what kind of godly gamer elements you want to add. It could be a system that gives the character super stats or unique abilities. Then, start with a strong hook, like the character suddenly finding themselves in the anime world with this new gaming power.