Remove all the hardware on the box using a screwdriver. In the case of very small boxes, you may need to use the screwdrivers from an eyeglass repair kit.
Sand the box lightly with fine-grit sandpaper.
Wipe the box down with tack cloth to remove the sawdust.
Trace each side of the box that you want to cover with wallpaper onto the wall paper, using a pencil. When you are done, you have a series of rectangles, one for each side of the box.
Cut out the wallpaper rectangles with a pair of scissors.
Wet the adhesive backing of the wallpaper and apply each rectangle to its corresponding side of the box.
Allow the wallpaper to dry completely. The packaging on the wallpaper will tell you how long this takes.
Reassemble the box, screwing the hinges back in.