1. Fix up Billy without the back board and without the middle shelf
2. Install “middle shelf” not in the middle but at an appropriate height for your desktop (I chose 75cm). Use screws and 4 L brackets to fix the self.
3. Align the edge flush to the edge of the board
4. Fix the table-legs under the board
5. Lay the board on the “middle shelf” and fix it there with screws from below
I eliminated the backboard because I otherwise had problems guiding my cables from the desktop down to the electrical socket.