Option 1: The simple answer is you don't have to prepare a file. Send what you have - text, a sketch or digital images - and pay for file preparation. If you are more comfortable using Microsoft Word or PowerPoint, set up your single-page document to the proper dimensions and save it as a PDF to pay only for printing.
Option 2: If you have computer graphics background, technical information on the following pages can be helpful. With an RGB digital photo of sufficient resolution, or vector art from Adobe Illustrator, there may be little else to do than complete an output worksheet and send your files.