For a good character interaction between Garou and Saitama, start with their fundamental differences. Saitama is a hero who is bored with the lack of challenge, while Garou hates heroes. Have them start with a fight, but make it a comical one where Saitama's nonchalance annoys Garou to no end. This can lead to some great dialogue exchanges where Garou tries to taunt Saitama into taking him seriously, but Saitama just doesn't care.
Writing a great 'naruto like garou fanfic' is no easy feat. You should start by deciding on the central theme. Is it about redemption like some of Garou's storylines or about achieving a great goal like Naruto? Then, think about the setting. It could be a new world that blends the fantastical elements of Naruto's world with the more gritty and urban feel of Garou's. When creating the characters, give them traits from both Naruto and Garou. For the plot, create a series of challenges that force the characters to use both Naruto - like and Garou - like skills. For instance, they might have to face a moral dilemma like Naruto and then fight their way out of it using Garou's combat skills. Also, don't forget to add some emotional depth to the story, so the readers can really connect with the characters.
Well, getting better at fiction writing involves paying attention to details and description. It's also important to study different writing styles and experiment with your own. And always revise and edit your work to make it the best it can be.