Go into your My Documents folder. and look for the Lord of the Rings Online folder.
Example: C:\Documents and Settings\User\My Documents\The Lord of the Rings Online
You probably have lots of screenshots in this folder.
Make a new folder called Plugins.
Find some plugins you would like to use at http://www.lotrointerface.com
Unzip the plugin and drag it into the Plugins folder.
Load the game up. You can now turn on your plugins from the character select screen.
If you are already in game, you can click the up arrow on the left side of the skills bar. Then HOVER over the system icon. You will see a new panel open with the plugin manager in the list.
You do not need to log out of the game to install a plugin. Once you have them in the folder, hit the refresh button in Plugin Manager.