I am using Ubuntu but the process will be same for any Linux distro or Mac OS
Let's get started, first open up Transmission and follow these steps.
Step 1: goto file menu and select New.. or just hit Ctrl + N
Step 2: A dialog will open up
give a location where the .torrent file will be downloaded (Desktop in my case)
and browse and select the Source Folder/File which you wanna seed.
In Trackers section you can provide some tracker urls from OpenBitTorrent
udp://tracker.openbittorrent.com:80 udp://open.demonii.com:1337 udp://tracker.coppersurfer.tk:6969 udp://tracker.leechers-paradise.org:6969
another dialog opens up
Click on Add.
the .torrent file has been created at the specified location (Desktop).
now open your .torrent file in transmission and start seeding.
Step 3: Finally goto your favorite torrent site and upload the .torrent file so that people can download it from there.