The boards are stacked together to make thicker dimensions. Generous amount of silicon are to be used. After which, pressure is to be applied so that the bonds are strong enough.
You can start carving your background after leaving the silicon dry for 24 hours.
My tank is with bracing so I have to break the background into smaller pieces. The pieces are painted with cement.
Note this space behind the background. It is for the powerhead to be placed in so that it can be hidden from sight.