Bring to class proposals and sketches for your storybook project. Know the stories! It sounds like a Doh! moment, but in book jacket work you have to read the text, especially for fiction. How else will you know that a pearl bead is a key icon in the tale?
Do lots of thumbnails, for the cover and for the interior. Doodle your illuminated initial too. Anticipate whether you want to do hand lettering (highly recommended) or set type. If your title is hand lettered it does not have to be calligraphy or script. Play. Invent letterforms. If you set all of your type, do you want to customize it? Consider display typefaces with a lot of personality that will communicate the tone of the stories and support the visuals.
If you aren’t an illustrator, consider other ways to create imagery for the cover and interior. You can do wonders with photography, found objects and images, a scanner and Adobe Photoshop.
The whole design should be a pleasing combination of type and image.