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This is, I hope, a very simple question.

Say that I use Torrent Creator to create a torrent with the start seeding immediately option unchecked.

Now, how do I activate this torrent (that is, start seeding) later on, at a time of my choosing?

Well, the program will ask you where to save the .torrent file.
When you just want to start seeding it, open this .torrent file and start seeding. That's about it.