This Shadow Talent Build is the fastest for Leveling Up a Priest Class from 1-60 in WoW Classic patch 1.12. Spamming Shadow Damage DOTs while kite mobs will be the name of the game with this build, having the ability to sustain your health is just a bonus. You will just need to make sure you are stacking enough spirit so you don’t run out of mana often as that will slow down the levelling process drastically.
After this past year of working on this project and forging our way through the various bugs and challenges, one consistent theme that’s emerged is that the difference between what we have and what we want is clearly visible. When we look at today’s World of Warcraft, we can see the differences between the modern game and the classic one. If we tried to update the reference client, we would have instead been tracking down a lot of “invisible” changes such as exploits waiting to be abused, crashes that don’t show up until you have millions of players online at once, and more. We chose to approach the problem in a way that makes our job clear and obvious instead of difficult and hard to see. 
You will have to talk with flight masters in most towns or cities you visit in order to gain access to that settlement's flight path. Flight masters who have a stop you have not yet unlocked will have a green displayed above their head (and on the mini map), much like a NPC with a quest. Unlocking flight paths as you progress through various zones will allow you to pay for a quick ride to a destination you have previously visited. This method of travel is significantly faster than walking or riding a mount.

My main plan will be to focus on raw gold in the beginning. Then I will spend my gold on leveling tailoring and enchanting so I can shuffle enchanting materials which is likely to be very profitable. I already have some shuffle ideas, and I will look for more. Then I’ll work on obtaining the rare enchanting and tailoring recipes to craft those for gold.