Instructions
Download the "Power Auras" addon. It comes compressed in a zip file. If you don't have a unzipping program, download and install one. "7Zip" is free and open source. "WinRar" has a free one-month trial.
Double-click on the "PowerAuras.zip" file to open it. Drag and drop the contents into your "World of Warcraft" addon directory. By default, the game installs to "\Program Files\World of Warcraft\Interface\Addons."
Start "World of Warcraft" and click on the "Addons" button in the lower left hand corner. Check the box next to "Power Auras" to complete installation. Once it's done installing, you now have to create the auras for each of your Shadow Priest's buffs.
Type "/powa" without the quotation marks. This brings up the "Power Auras" interface menu.
Click on an open slot and then push "Import."
Minimize "World of Warcraft" and open a browser window. Go to the Shadow Priest page of the "Power Auras" wiki.
Highlight the import codes and right click on them. Select "Copy."
Return to "World of Warcraft." Click on the import box and then press "CTRL" and "V." This pastes the import code into "Power Auras."
Close the "Power Auras" window. From now, auras will surround your character when Shadow Priest buffs are active.