Well, initially, get to know Kakashi really well. Consider his likes, dislikes, and his typical behavior. When it comes to the yaoi aspect, determine the nature of the relationship, whether it's more passionate or a slow - developing love. For the lemon elements, you should be careful as it can be a tricky area. You can start by writing some character sketches and dialogues to get a feel for how they will interact. Plan out the story arc, starting from the introduction of the characters to the climax of their relationship, and finally, the resolution.
First, you need to have a clear understanding of the characters, especially Kakashi. Know his personality, his backstory, and his relationships in the original series. Then, think about the relationship dynamic you want to create between him as the uke and the other character. Sketch out a basic plot, like how they meet and what kind of situations will bring them closer.