This option is better to use at level 40+. Higher level you are is better, more money you make then. All you do is farming dungeons or specific bosses. Here you kill a few birds with one stone: you get lots of experience (make sure you have all quests in your quest log, always ask your teammates to share) if you are still leveling; improve your current gear; and loads of loot (clothes, recipes, even crappy grey items selling for a lot to a vendor). If you are in a group, think about your groupmates if they need an item or not, don’t roll need on everything.
One may think that the AH may suffer a reduction of competitivity, leading to higher prices, but in an healthy server the gold farmer's farm will be taken over by actual players. Giving the chance to a more fair gameplay where you can go out and farm the materials that you need, instead to see them always farmed by gold sellers that sell them to you, without letting you many room to make gold in a fair way.
Through out the game, the quest log will be capped to 20 many times.  The guide makes use of maximizing the total amount of quests you can accept at once.  Because of this it is important to follow the guide exactly and only accept quests if the guide tells you to so you won't run into issues not being able to accept a quest.  If you do accept a quest that is not listed in the guide, write it down because you may need to abandon it in order to keep following the guide if your quest log is full.
The quickest way to level up is to travel through the game's zones, completing all the quests you encounter along the way. Completing quests grants you Experience points, which will slowly fill in your experience bar with blue or purple. When the bar fills, you reach the next level. Alternatively, you could choose to run Dungeons or Battlegrounds, which are not covered in this beginner's guide.