Could not be any easier. just get 2.5 inch SATA Hard Drive to USB 2.0 Adapter Cable with Silicon Protective Case connect the drive to a usb port. Go to the seagate site download and run their Drive wizard and follow the directions.
No, you don't need to purchase a new usb2.0 cable. The usb3.0 cable included in the package will either fit or work well with your usb2.0 or usb3.0 connectors on your computer. And I don't think you can use a 2.0 cable in case of you have or want to buy one, because the 3.0 header on the HDD, which will connect to the cable, is different from the old version for usb2.0.