Prepare a simple text page with your name, the product and the word SPEC AD in the upper right hand corner. Be sure to include the words "SPEC AD" because your goal is to show a potential client or employer your talent and not to deceive them into thinking you really worked with this particular client.
For example, if you were using an original Kraft Foods print ad as your SPEC AD, leaving off the words "SPEC AD" leads the potential client/employer to believe you worked with Kraft Foods and its agency to create the original.