And now, last but least, the attention catcher and main reason why I stick to 抖阴社区, the Cover!
Now, cover designs could be seen as a form of visual art. And teaching Art though text is a bit complicated. So, here is all I can say.
Unless your cover isn't just a big stylized text on a background, many YA books have such covers, the middle should be an image that fits the tone of the story. This attention catcher may be a couple for a romance, gem for a Fantasy story or a star ship. It comes into the middle.
The texts, keep it simple, and low. Title, maybe a subtitle and who writes it. And maybe, if it is a transition, the name of the translator.
You should pick a font that fits the story, but you should prioritise that the text is readable. Which also means it should be big enough, but not so big that it takes the whole cover space.
The background and Text should have a strong contrast. No similar colours, they would melt together and not be very readable from afar or when the cover is small.
Your goal is, if you turn the cover black and white, that the image and text are visible. The shapes of the image can be seen, it should he recognisable as i.e a starship and the text should be readable and a different shade of grey, black, white, anything that is contrary to the image and not blend in. This test is simple.
If you can't make a cover or just don't want to, don't just take a fan art from the web and Slap it on. Maybe resch out to a cover shop on 抖阴社区? Or ask ABSOL_ute to make covers. She offers on r/抖阴社区 to make Fanfiction covers.
Though, her style is non-realism minimalist. Check out her Cover art book, especially the later chapters/covers.

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