frame the featured art for your cover so that it looks nice. man, this is going into art theory or whatever but usually you want it slightly off-center somehow so you can balance its "weight" with the title text. that means you also want negative spaces where the reader can "breathe" and focal points where their eyes will shift to first.
pick bold colors that stand out. (did you notice a lot of movie covers use orange and blue?) make sure they don't clash (for god's sakes don't put neon green and pink next to each other unless you want to blind us all) and can be distinguished as distinct shapes if you squint at it. remember SH covers are tiny so you don't want readers to see your cover and think "wow what a load of blotches"
typeface is an art and fonts have personality. pick ones that suit the mood of your story best. remember that when put on a cover, words become part of the "picture" so space them out accordingly.
uhhh you can check my covers if you like, i tried to follow my own principles there. (SH's pictures are tiny tho, they look much better on their Wattpad versions)
disclaimer: just an amateur talking, take it with a grain of salt
