Why would you need a Custom UI
The UI in AoC is simply not enough for most people considering the amount of skills and abilities available in the game, the default UI only has 1 skill bar that you can see at a time while a custom ui can have upto 4 times that amount which allows you to bind and see all your abilities and skills at once.
Things you will need:
AOC UI Installer
A Custom UI of your choice
AOC UI Installer
AOC UI Installer can be found on the AoC forums or a short google search will give you the download.
Once downloaded, install it using admin rights and once install run it as administrator.
In the "paths" section, find your Age of Conan folder and we're ready to go.
Custom UI
The only place at this time to get a Custom UI is on the AoC Forums under "UI & Mod Customization"
Stonerune, Corruption and Strange UI are a few of the really popular ones.
So head to the official AoC forums and choose which one suits you best, all of them have different variants as well as pictures to show you how they look in game. They should all provide a zip file as a download, download and leave it where you will easily find it since we're going to be using the zip file in the next step.
Installing the UI
Open up AoC UI installer if you haven't already.
Select the zip file you downloaded for the UI you want to install.
In the "Select Bar Size" section, check to see if there's any other options you want installed.
Once you selected the variants you want, click install and let the magic happen.
Patch UI Files
Once installed it is a good idea to just hit the "Patch UI Files" button in AOC UI Installer.
If a UI is really old it usually has to be patched.
It also usually needs to be patched after major updates to the game.
So If you ever go in game and you get a bunch of errors and you've somehow switched to the default UI, this means you need to run AOC UI Installer and just hit the "Patch UI Files" button.
How to see my new UI
If you're not in game you can just load the game client and it should be there once you've logged into a character.
If you're already in game you can type /reloadui into the chatbox and it will reload you're UI.
If you've done everything correctly then your brand new UI should be visible for you.
If you get a ton of pop up boxes then you need to patch the files.
If you get absolutely nothing then you have probably not given AOC UI Installer the correct path for Age of Conan.
Other Issues
You can always have a read of the download page for specific UIs which usually have a ton of troubleshooting tips and it's more than likely that someone is having the same problem as you.
You can also make a new post on the official forums and someone will usually help you.
If you have any questions feel free to reply to this post and I will answer if I'm able to.