Here's the NASA article on this:
But my concept on how to do this is different and has the advantage that it could be done easily by solar-powered robotic equipment. (Maybe their method of construction would be mostly automated, as well.) My thinking on building any kind of massive structures on the moon has been to do selective laser sintering with moon dust, which would create monolithic moon rocks of any desired size and shape. This method has been used on Earth to create objects first of aluminum but more recently of various metals, building solid metal object from metallic dust. With moon dust, you would end up with rock, basically, and it would be a bit radioactive. The last stage would basically be the same as theirs, in the case of a telescopic mirror: coating and polishing.