Select your target, cast from 30 yards away. Use Mind Blast first, then the target will run towards you, use Shadow Word: Pain, Devouring Plague and Vampiric Touch. The target should have now multiple dots and debuff. Then carry on with Mind Flay. Don't forget to use Mind Blast again when its CD ready. Use Shadow Word: Death as final strike, if the mob is still not killed, add one more Mind Flay.
Shadow Word: Pain
Shadow Word: Death
In conclusion, it's pretty simple for Priest to kill mob at outdoors. General throw a bunch of DOT on the mob and use Mind Blast again when CD ready, use Mind Flay at most other time, add Shadow Word: Death if necessary. If you still have enough HP like 70%, carry on with next target, otherwise change to Shadowform, and use Greater Heal to recover yourself.