All people would need to do would be to produce a machine that could build a copy of itself from atoms. I suppose it might also need to be able to do some sort of fusion to convert gaseous elements into solid matter as well. This machine would also need to be able to escape a gravity well such as a planet. Each machine would need to be programmed to then produce copies of itself, which also would be instructed to do the same thing. These machines would not necessarily need to be capable of faster than light travel, although that would certainly make the process quicker. Oh, and these machines would need to also be instructed not to consume copies of itself when producing new machines. This would end with the universe being full of nothing but these machines. The end.