hello! I don’t really have any advice since my works are just made because of boredom hahah but If I have to tell you something it would be, Let your mind and imagination do its work. Always remember that what you do is for yourself not for others. Find inspiration, there will be times that you’ll get tired, You can take a rest, you are not obliged to finish your work on time.
Your work, Your Ideas, Your rules.
Remember to have fun and good luck on your first fan fiction !!