+White paint
+Screwdriver
Start by cutting 10-12 small squares 7x7x7 cm and one 33 cm long piece from the wooden beam. Paint them all white.
Create two modules made of two cubes glued together, and two modules made of three cubes like in the picture. To make the construction stronger you can use nails, but a strong wood glue should do the trick.
Glue the modules together like in the picture or create your own creative version with the cubes. Paint everything again if some flaking or chipping has occured during the construction. Screw the loops onto the back, use at least two. Use a level when hanging if you want the shelf to be completely straight.