I guess the best way would depend on your budget. Buses might take 15 or so hours Cusco-Nazca (I'd recommend Cruz del Sur). I don't know whether there's a private taxi service that does this trip but considering the distance it's probably expensive.
You can't fly directly from Cusco to Nazca... You could fly to Lima and take a bus or a tour to Nazca and then fly over the Nazca lines or you could charter a plane from Lima (expensive but the option exists, this tour site has details). Apparently you can also fly from Ica or Pisco... There are other flight operators but I can't recall their names.