Take baby steps.
1. Start with the basics you learned in elementary school (drawing circles and other shapes, straight line) you want to be able to draw those with confident lines aka not shaky, not dozens of lines, being able to do them asleep
2. look at references, references, references
3. Do studies to each topic (Anatomy, gesture drawings, artstyles from different artists, perspective, color theory, COMPOSITION)
4. Draw with intention and thought. You don’t want your cover look out of place. Character’s pose, colors, placement, and overall design; why would they cloth like this? What does their posture and expression tell you about their personality? Question your stylistic choices.
5. Do thumbnails of several ideas, don’t settle for the very first idea. You might come up with a better idea or improve your idea with this simple brainstorming.
6. Practice to get good.
there are also tons of free stock photos you can use for photo bashing
Many anime artists start off instantly with stylized drawing but it’s equally important to do realistic gesture drawings. Irl is still the primordial reference of anime and your anatomy improve the best if you can understand realistic.