Using your desktop publishing programme construct a simple line box. I keep the lines very narrow because i don't want them to show on the finished box. I also put longer fold lines in using a bright colour. This helps when you come to fold the box. The front and back panel of this box measures 13mm x 9mm, the side panel 13mm x 4mm and the top and bottom flaps 9mm x 4 mm. I add an extra box on to glue the side of the box to the back. Pretty straightforward really, the sizing to scale is important also the accuracy of the template ensures a nice finished box.
By adding longer fold lines to your template it makes the folding of even the tiniest box alot easier.