First of all there probably isn't a "best" way to make gold in WoW, there are however, probably hundreds of ways to do it. Here's a list of some of the easiest and most common ways though:

1. Train a gathering profession (mining, herbalism, skinning) and sell the mats
2. Complete daily quests for gold
3. Finish off quests you didn't complete while leveling from 1-70
4. Sell all vendor trash
5. Sell greens and other worthwhile items on auction house
6. Farm items like gear, reputation items, primals, and other stuff and then sell on auction house
7. Fish and sell fish on auction house

Like I said those are just a few of the ways to make gold, just pick whatever suits you best.