The creation of a mockup may be as simple as a pencil and paper sketch, or a design rendered in a product such as Adobe Illustrator or Photoshop. The benefits of using a software tool is that once you have a mockup the client has approved, you may easily turn it into a working model.
Simple pencil and paper sketch.
Photoshop sample with a few placeholders.
Electronic mockups are also easy to share and store. Make sure to use a format that is compatible to all involved and to protect the integrity of the design. Consider using a watermark or other security process to ensure the design is not able to be used until the time it is needed.
Demo - Using Freedcamp.com to share files: