Cloning of mammals was first achieved by Ian Wilmut, a research scientist working at The Roslin Institute, which is a leading international center for research on farm animals. Its research on quantitative genetics, genome analysis, animal physiology and behavior is aimed at improving the productivity and welfare of farm animals.
Nature creates clones all the time.
But at some stage during its growth, while still in the womb, the cells lose that ability because some genetic switches are flipped off or on. They become any one of the multitude of specialized body cells, and won't revert. That's convenient - it means when you cut yourself, you don't get a baby growing over the cut, you get skin. That's what Ian Wilmut and the researchers at the Roslin Institute in Scotland seem to have worked out. Then you take the DNA from the nucleus and implant it in an egg.