First of all, thank you for wanting to seed ROTJ.

I use Torrentstorm as my torrent downloader client. In it, I just open the torrent file in it. If it is completed, it automatically seeds it.

For bittorrent, I'm not sure how. I would try opening the torrent file in whatever client you used, and if it's completed it should automatically start seeding it.

Thank you for wanting to seed ROTJ, as it is loading the slowest of the three on my computer at a pathetic 2 KB/s.

EDIT: Ironically enough, I was the one who posted the link to torrentbox at the officail site.