1) Install Transmission using "yum install transmission" as root.
2) Go to System -> Administration -> Startup Applications
3) Add a new entry to the Startup Programs list: Name = "Transmission"; Command = "transmission-gtk %F"; Comment = whatever.
4) Open Transmission and go to Edit -> Preferences.
5) Set everything up as you want, but there's a few specific options that must be set. In the "Torrents" tab, tick "Automatically add torrents from" and choose a folder in your shares that you can drop torrent files into from other PCs. Tick "Start when added", "Move .torrent file to trash" and untick "Show options dialog".

In the "Web" tab, tick "Enable web client". You might want to tick "use authentication" and choose a username and password too.

6) To control your torrents you have two options:
a) Go to "http://hda:9091/" in a web browser. Replace "hda" with your HDA's IP address if you haven't set up DNS etc.
b) Install Transmission Remote GUI from here on all of your network PCs that you want to control your torrenting from.
Option (a) is simpler but option (b) gives you far more control. If you went for option (b) there's some more steps!
7) Open Transmission Remote GUI and go to Tools -> Connection Options.
8) Make a new connection and call it whatever you want. Set "Remote Host" to "hda", "Port" to "9091", and enter the username and password you set in step 5. Leave these blank if you didn't set a username and password.

9) In the "Paths" tab, enter any path mappings you want. For example, "/var/hda/files/torrents=\\hda\torrents" will let the program properly access the local folder "/var/hda/files/torrents" by accessing your "torrents" share. This is required for things like "Open containing folder" to work.

10) Click OK and then go to Tools -> Options.
11) Customise this as you wish.
12) Go to Tools -> Transmission Options.
13) These options should match what you set on your HDA.

Now if you want to add a torrent, there are 3 methods:
a) Copy/download a .torrent file to the folder you specified under "Automatically add torrents from" in step 5.
b) Open a .torrent file with Transmission Remote GUI.
c) Open a .torrent file (or enter a torrent URL) in the Web Interface.