Shadow priests can choose any profession they like, but there are some that make sense considering the nature of the class. By default, you’ll need several potions and flasks to help empower your character. Additionally, mana potions are often actually a part of a shadow priest’s rotation considering their mana issues. This makes alchemy a natural choice, but there are a few others that will also help you out.
The Horde 12-20 (Barrens + Stonetalon Mountains) guide has been rewritten and revamped.  In addition, there has been numerous tweaks to the speedrun route to make things faster and easier to follow.  Also, in case you haven't noticed I am now adding about 40% more info per step and redoing all the images.  Unlike with my previous vanilla guides, I am now giving a brief explanation of where to go and what to do with each step along with any other helpful tips.
Spirit increases one’s regeneration rates. The amount of health generated by Spirit outside of combat is almost negligible, but the mana regeneration granted by Spirit is nothing to scoff at as a priest. Spirit Tap doubles your spirit for 15 seconds after getting a killing blow, and allows 50% of your normal mana regeneration to continue while casting. This is very important for priests who are soloing or are leveling.
Cone of Cold is behaving consistently with the reference client. Arcane Missiles does not put the caster in combat . A Hunter’s Frost Trap ground effect will break Rogues out of stealth. The Berserking Troll racial ability is behaving as expected and matches the reference client. The pet that a Warlock has when initially logging into the game world does not restore a Soul Shard when dismissed by taking a flight path or moving out of range. The trigger range on Hunter’s traps are reduced by Stealth when the stealthed player is a similar or higher level than the Hunter. Soul Link cannot be dispelled by dispelling the Warlock’s pet. A Warlock’s Succubus pet cannot cast spells if they are out of line of sight of the target. Manually cancelling Stealth after using Vanish will remove the Vanish buff as well as the Stealth buff. Escape Artist has a very small chance to fail when used to escape an effect that has a decreased chance to be dispelled (e.g. a Rogue’s Vile Poisons talent). Rogues are not broken out of stealth by Blizzard until they take damage. Taunting Hunter pets that are set to Aggressive or Defensive mode will cause them to attack the taunting player. I additionally modified the wording on the following to be a bit more clear:
If for any reason, two or more mobs are pulled, apply Shadow Word: Pain to all targets, then use Psychic Scream to reduce damage taken. Apply Vampiric Embrace as necessary to heal any damage. Keep calm and shoot that wand. Re-apply Shadow Word: Pain as necessary. If mana is not an issue, cast one or two Mind Flays to kill a target quicker so more of the fight is spent with Spirit Tap active.
It’s important that you levelling in certain areas around your characters level to gain the full XP reward for slaying monsters. Below is a list of all the best location for grinding XP from level 1 to 60, these locations are good for AOE spells and should have plenty of mobs. The Level you should be before you enter that specific location will be indicated to left in a Blue Color for Alliance [Lvl 35-39] and Orange Color for Horde  [Lvl 35-39] the Mobs you are looking for Bolded [Mob Name].
Now that you know how to optimize your shadow priest, you’ll need to understand how to actually play one. While shadow priests are technically a supportive damage-dealer, they don’t have as much responsibility as hybrid classes like a retribution paladin. You don’t have buffs to maintain, and your support comes in the form of passive bonuses you apply.
While it is possible to play perfectly with the default UI, there are many critical things missing that will make your life more difficult. This is where addons can be extremely useful. There are a vast number of addons, including more comprehensive UI packages, but below we have listed some of the basic addons that are highly recommended as a Hunter.