This is not really about writing, but unless you post solely on AO3, you know how important this is. A good cover art will help you attract readers and make them read your summary.

I usually start working on the cover art while writing the story. It helps me stay in the mood when I need a break from writing, and it also helps me to set the mood of the story. 

I like to make covers that convey the theme of the story, rather than putting the characters’ faces, and I will usually play with a few different concepts to see what works and what doesn’t, before deciding on a final cover.

I have some knowledge in design and digital art, but I don’t think you should be afraid to try and do your cover art even if you have no background in that field. There are websites like Canva that are really intuitive that should help you. If you don’t feel up to it, you can always ask someone to do it for you! Just remember that you should never pay or receive money for anything related to fanfiction. 

Small quick note: I am available if you want help with your cover art.