One piece is probably missing.
Try from command line:
systemctl --user daemon-reload
systemctl --user start transmission-daemon
If that works, you can then, if you like, do
systemctl --user enable transmission-daemon
this should start the daemon each time after rebooting (I think haven’t tested). But that’s probably not a good idea, because of bandwidth usage you may pay for.
Just use one of these when you want to start and stop it:
systemctl --user start transmission-daemon
systemctl --user stop transmission-daemon
That’s as far as I can support you, you will have to figure out the rest of how transmission works by yourself. The config file is found in ~/.config/transmission/settings.json
(as are some other dirs, such as the torrent dir).
For the rest, read the documentation of transmission:
https://forum.transmissionbt.com/
https://trac.transmissionbt.com/wiki/WikiStart