Human cloning is the creation of a genetically identical copy of a human being (not usually referring monozygotic multiple births), human cell, or human tissue.
Put simply identical twins are clones of each other and share the same DNA. Therefore to create a human clone from an adult just means taking a cell from that person and triggering it to begin life again as was present at birth.
The problem is cells age with time so the clone will start life at the point reached by the donor. They found this to be the case with 'Dolly the Sheep' which developed illnesses only found in older animals.