Some products have especilly prepared instructions and hints for designers. You can read the technical specification as well as how to prepare files properely to printing.
On some of the products, especially those which are more sophisticated, you may find examples of files. These are files well prepared for printing purposes.
On most of the products you'll find ready to use templates. They're very useful things when working over the artworks by yourself. Prepared and saved as different file types for your convenience. On templates you'll find cutting lines, bleed areas, creasing lines, etc. Just download, open in your graphic software and design!