Complimentary DNA strands are made using starter primers & dideoxy-nucleotides. When ddNTP's are incorporated, it terminates a growing  DNA strand, thus we make many strands of different lengths, each with an end nucleotide fluorescently tagged (different colors).  All the DNA fragments undergo PAGE through capillary tubes, which separate the DNA strands by size length. They are then scanned by fluorescent detectors and plotted.