If you want to learn how to unlock flying in Dreaenor, check out this guide. This is a somewhat time-consuming achievement, but will make leveling trough the WoD zones a breeze compared to non-flying.
Check out all parts of the leveling guide:
Part 1) Level 1 - 58 / vanilla leveling guide.
Part 2) Level 58 - 70 / The Burning Crusade leveling guide.
Part 3) Level 70 - 80 / Wrath of the Lich King leveling guide.
Part 4) Level 80 - 85 / Cataclysm leveling guide.
Part 5) Level 85 - 90 / Mists of Pandaria leveling guide.
Part 6) Level 90 - 100 / Warlords of Draenor leveling guide.
Part 7) Level 100 - 100 / Legion leveling guide.
Level 90 - 100 with pots and flyingWith some spare gold and flying unlocked, you can actually level from 90 to 100 in less than three hours, but it require some preparation. What you need to do firstly in to level trough Shadowmoon Valley / Frostfire Ridge until you unlock your Garrison. From here on it's time to head to the different bonus objectives around Draenor and complete them to as close as 100 % as possible without triggering them. This step will probably take a few hours.
For the next part you need to purchase 1 x Elixir of the Rapid Mind from the auction house. This usually costs around 20k gold on most servers. Also buy 1 x Excess Potion of Accelerated Learning from your Garrison Quartermaster for 100 Garrison Resources. Once you have these two as well as all bonus objectives at around 99 %, head to a random dungeon and pop the pots right before the boss gets killed. Now turn in the quests, and this will give you a few levels.
Once you are done with the dungeon, run to all the bonus objectives and kill the final mob needed to get the experience bonus. With the pots, these will be worth a ton of XP, and you will hit level 100 before the 15 minute XP potion buff expires.
Level 90 - 100 with flying, without potsIf you got flying, but don't want to spend 20k on Elixir of the Rapid Mind, the best way to level up is simply to fly around and loot all the hidden treasure chests in Draenor. This might be very boring, but it is somewhat efficient for leveling. If you want a break from this, sign up for a few random dungeons.
While working on picking up treasures, complete all the bonus objectives on the map, as well as kill any rares you come across.
Level 90 - 100 without flying and potsIf you cannot afford the Elixir of the Rapid Mind and don't have flying unlocked, you are in for a much longer ride trough Draenor. What you want to do to level up as fast as possible is to simply rush the quests while always staying queued for a random dungeon. Make sure to do each dungeon at least once for the massive experience from the dungeon quests.
As with the other methods above, always complete the bonus objectives for a nice bonus XP. These are really nice for leveling up!
Stay away from the scenario quests, because they are not worth it time-wise, even though the give a nice amount of XP after completion.