The location of files and folders is critical and is many times the part that gets looked over as difficult to understand. If you are not familiar with using Windows Explorer (My Computer icon) to navigate your computer, the following may even assume too much.
Double-click the 'My Computer' icon and navigate to the folder where you downloaded the UI file posted on this website.
We will assume C:\My Documents\FetishNightFall_9_9b.zip for this instruction.
Double-click FetishNightFall_9_9b.zip to open the built-in ZIP file handling window in Windows XP.
Click 'Extract All Files' on the left window bar as shown above.
Navigate using the Browse button until you have located the directory where you have installed the game. This is probably similar to the file path shown in 'Select a Destination' dialog pictured below.
If you found the correct directory and clicked on OK on the dialog, then click Next on the window shown below and then Finish to complete and close the window.
If this process completes without errors you should be ready to run the game and try out your new UI.
Backup is always a good idea if you are unfamiliar with changing your computer files and certainly if you are not comfortable reversing what you have done without the assistance of software.
I hope this helps.