As mentioned a few days ago:
Seeding is just like doing a download. You just point bittorrent to the folder you have already completely downloaded as if resuming. It will say "checking files", then "Download succeeded!". That's it! You will be seeding downloads for as long as the window is open.
Seeeding has the potential to start an exponential growth process that benefits a lot of people for a long time. You'll notice if you look at the tracker page that most episodes sets either have many seeds or none at all. That's because even a single seed will help others to complete, and if their windows stay open they become seeds themselves, so others complete even faster, etc., so the whole process builds on itself!