Let me type this out:
*(1) You have to download the software [BT]
*(2) Well, simply install it. If you want to set up the properties, you can do that when you opened a torrent. (I am using [bittorrent8GB.exe], the very simply version. You can find something more fansy in the web.)
(3) Please download the torrent if you prepare to shutdown the connect when you downloading.
(4) Re-open the torrent and save it in the same place that you saved your orginal file. It will self-detect the file size and start to re-download from that %.
EASY! You are done by now. This is the simplest form to re-download.