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I have my Transmission set to Automatically start any Torrents found in my Downloads folder. Preferences > Transfers > Auto add

Once I've downloaded a torrent (legitimate of course!), Transmission simply asks what parts of the torrent it should download.

It's my preferred client due to it's ease of use!
 
After you download a torrent, drag it into the Transmission window, another window will pop up, check the "start when added" checkbox and then click "Add". The torrent will then begin downloading.

Once it has finished downloading, leave it in Transmission until it finishes uploading (seeding). If you download 100MB... make sure 100MB is uploaded. Bad things happen to those who don't seed.
 
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