Hi, I felt like making a guide just because I feel like a leacher so here is my guide to downloading a UI.
1.
Visit the sight WoWInterface Downloads and pick a sub catagory depending on your style.
2.
Ounce you have found a UI *I'll use Kwi UI as a example* look at the description and the pictures the user provided and make sure it is the one you want!
3.
Make sure you have Win.RaR or another extraction device and hit "Download" Then extract the folders to your desktop.
4.
Make a copy of your current Interface/WTF folders any where of your choice, then go into the WTF folder YOU DOWNLOADED and rename the 'YOUR ACCOUNT NAME', 'YOURSERVERNAME', and 'YOUR CHARACTER NAME' folders to your account; create multipiple fodlers for all your characters/accounts/servers.
5.
Extract the files that you downloaded into your wow folder *make sure your old interface/WTF/and font folders are moved to a different folder*.
6.
Log into World of Warcraft and make sure everything went ok, then log into one of your characters and look at the page you downloaded the UI from for in-game instructions if necessary