Write down key points you want to include and also make detailed notes as you remember everything so you don't overlook a key point in the story. Then write it out slowly one chapter at a time while referencing your notes. For me it was extremely easy to see myself as Lucy and my love as Natsu simply because the personalities matched, that's why I write a lot in first person view be it X reader or Lucy because I can easily see myself as the character.
I'm still learning a lot about writing myself so I don't consider myself the best at all, I just have a very active imagination and a passion for writing and telling stories.
I hope this helps, if you need any help feel free to comment here or private message me if you want and I'll try my best to help. 😁❤️