Spray adhesive on one inside cube wall and the reverse side of one wallpaper sheet, position it to fit along the front then smooth towards the back. To straighten the pattern, pull paper from the back and re-position, working quickly before the adhesive dries, flipping the cube as you go for easy access. Repeat for other walls.
Safety tips: When spraying adhesive, wear gloves and a mask and check the weight allowance of the shelves before placing items on them.