1: Go to the following Path:
C:\<YOUR INSTALLED DIRECTORY HERE>\Electronic Arts\Dark Age of Camelot\ui\

2: Add the Custom folder to it. If you already have a custom folder, drop the .bmp files into it.

3: Log into the realm selection screen, and press the "Options" button.

4: Under the "Interface" section, make sure "Custom Skin" is selected.  

5: Click accept and play as normal with UI buttons that make sense.