Find a nice, royalty free, graphic for your base. Pick a rectangle with a landscape orientation. You can uses something that fits your theme, a product, or a solid color.
Upload your graphic into your editor.
If your editor allows you to hide layers, such as PSP, this will be your background, go to "Making Sections"
If your editor does not allow hiding layers make a solid layer for the background and the base layer should be slightly smaller so the background looks like a border.