When a torrent is finished downloading, of course not stopping the torrent by clicking the 'Stop' button results in the file being seeded. If you already stopped the torrent then to start seeding again all you have to do is click 'Start.' That is, of course, unless you removed the torrent from the client some time after it was finished.

To get it back in seeding condition isn't always a cut and dry process since it isn't always technically the exact same as when you downloaded it to begin with. With those private system messages though, it's not like you're *required* to seed that torrent at that exact moment or something will happen to your account. It's just if you're there and you're available to seed that file since someone may be trying and is having trouble finding seeders.

I hope that helps. Unless you already knew all that and were indeed asking how to get the files back onto the client for seeding after you've already removed the torrent, in which case I don't have enough experience nor knowledge to sufficiently answer that.