Share the book’s premise, intended readers, comparable titles and the feeling you hope the cover will create. Note required wording, series details and imagery that must be avoided. If you have visual references, explain what you like about each rather than asking a designer to copy another cover.
A focused brief gives the designer context without prescribing every element. Agree on the number of concepts, revision rounds and final deliverables in advance. Clear communication helps the cover represent the book accurately and work at both full size and as a small online thumbnail.