After writing your design brief, you name the contest's cash prize (that's the design package you chose), and publish the contest on the design marketplace. Subscribed 99 designers from around the globe will then be able to view your specifications and start designing.
If you really love the work of a particular artist, you can work with him or her one-on-one on future projects.
(And if you'd like to try designing your own book covers, check out my 5 tips for better book cover designs.)