Start by creating a Photoshop document for the front and back.
First:
> Launch Photoshop.
> Select <New> from the
<File> menu.
Then specify the size of your new document:
> Create an image canvas that is 9.5 inches wide (24.14 cm) and 4.75 inches high (12.07 cm).
> For the mode, select
<RGB Color>.
> Click <OK>.