Put nails in each 3 mm hole. Solder them together on the inside of the cubes.
Each nail of each side represents a negative or positive connection. Starting from the upper left corner you've got positive, on the upper right corner you've got negative, on the lower right corner you've got positive and on the lower left corner negative. Use electrical wires, solder and a solder iron to connect the positive corners together and to connect the negative corners together.