Step 1: The model 240 cm by 240 cm
Step 2: The materials
4 sticks of pine of 240 cm by 5 cm by 6 cm (5 is front; 6 is deep) (photo1)
6 selves of DM of 240 cm by 22 cm by 3 cm (photo2)
4 wall metal peg screws to hold large weigths (one for each stick) (photo3)
12 regular wall plastic peg screws to hold medium weigths (three for each stick) (photo4)
Step 3: Cut off mortices on sticks and shelves
In the sticks make a mortice for every shelf.
In the shelves make a mortice for every stick.
In our case this means 6 mortices in each stick and 4 mortices in each shelf.
As a rule, in sticks make a mortice 2/3 deep of the side of the stick and in shelves make a mortice 1/3 deep of the side of the stick. (In the example this means 4 cm in sticks and 2 cm in shelves because the side of the stick is 6 cm).
Step 4: Hold vertical sticks on the wall
Check verticality with a bubble level.
Fix the other 3 screws in every stick.
Step 5: Finishing
BUT THERE IS STILL ONE MORE THING...
PLEASE DO NOT MISS THIS DETAIL WHETHER YOU THINK IS IMPORTANT OR NOT!
The last thing to do is to insert wedges of wood under all the shelves and in every stick untill all the shelves are ortogonal (=perpendicular) (=90 degres) to the sticks.