Well, to begin writing your harem lemon fanfiction, you need to do a lot of planning. Think about the genre you want to incorporate, like romance, adventure, or comedy. For example, if it's adventure, your main character could be on a quest and meet the harem members along the way. Then, focus on character development. The harem members should be diverse in terms of looks, personalities, and goals. Also, when it comes to the 'lemon' aspect, make sure it's tasteful and in line with the overall tone of the story. You don't want it to be too vulgar. Start with an outline of the main plot points and gradually build on it as you write.
First, come up with an interesting concept for your story. Decide on the setting, whether it's a fantasy world, a modern city, or something else. Then, create your main character. They should have some unique traits that would attract others. Next, start thinking about the members of the harem. Each should have their own personality and backstory.
Begin by brainstorming ideas for your harem. What kind of characters do you want in it? Are they humans, or could they be from different species like elves or vampires? Once you have your characters in mind, think about the relationships between them and the main character. For the 'lemon' part, if you choose to include it, consider how it will enhance the story and the character relationships rather than just being there for shock value. Develop a writing schedule so that you can consistently work on your fanfiction. Read other harem fanfictions for inspiration, but make sure to put your own unique spin on your story. Start small with a simple plot and gradually expand as you gain more confidence in your writing.