Bitten's SpellFlash: Priest replaces Blizzard's default proc highlighting to instead flash a maximum dps rotation for Shadow and some helpful flashes for Discipline and Holy. You can see all the gory details of the rotation using those links, or the one on the navigation bar to the left. Download the addon from Curse.com. If you are having any troubles, check the FAQ and if that doesn't help create a ticket. If you have a question that the FAQ doesn't answer, leave a comment on the download page. If you have a suggestion or request to improve the addon, add it or vote for it on the suggestions page.
Bitten's SpellFlash: Atonement is a separate addon to use for easy content when you are a healer but want to contribute as much dps as possible. You can download it here and see the details here.