There are
two ways to do this:
...by leaving your client open after the download completes. Once you have the entire file you become a
seed, and the BitTorrent client remains connected to the swarm, sending to other users until you close it.
...by clicking on a torrent link (or opening a saved .torrent file) and selecting a filename of a file that has already completed. BitTorrent will
check over the file and realize that it's already complete, and continue to connect to the tracker and serve as a
seed.