The only requirement is that the work contain at least a nominal amount of creativity. Searches are not performed prior to the filing of a copyright application because if someone else has already written the song or book you wrote, it makes no difference to the Copyright Office as long as you did not copy from the earlier artist. A work registered more than ninety days after publication, and after infringement begins is still protectable, but the measure of damages is actual damages, and the artist must bear his or her own legal fees. A lawsuit to enforce a copyright on a work that was registered within ninety days of publication or more than ninety days from said date but prior to the onset of infringement may result in an award of statutory damages and attorneys' fees. Actual damages are hard to quantify, but statutory damages are substantial and need not be quantified.
Prompt registration is therefore highly desirable.
Actually, you would mail in a copy or photograph of the napkin as the attachment. If your sketch has not been published as of the date you mail in your copyright application, only one copy of the work will be attached to the application. If the work is published, then two copies of it must be attached to the application, and the date and nation of the first publication must be revealed. Unlike a patent application, a copyright application is a form that can be filled out without the assistance of a lawyer, in most cases. However, there are a number of pitfalls into which a novice can descend, especially if the work is a compilation or a derivation from an earlier work.