To start off DNA is copied in Interphase. Interphase happens before Mitosis. The first thing that happens during interphase is the cell grows to its full size. Then the DNA gets ready to divide. When the DNA is ready it splits down the middle. Now you have two strands of DNA.  
     There are many steps after that, but interphase is  the only phase where it is all about the DNA being copied. So those are the steps as to how DNA is copied. That is also the phase it is call when the DNA is duplicated.