Felweed is the easiest herb to farm, as it is the most common and is present in the most zones. However, the best place to farm it is in Hellfire Peninsula. Since it is found throughout the entire zone, the best route to take is to circle around the perimeter of the zone and then take two or three passes through the middle-section of the zone. This will allow you to obtain virtually most nodes of the Felweed present.

Ragveil is also fairly common, but only found in Zangarmarsh. If you need to farm this, the best route to take is to divide Zangarmarsh into three sections, east, west, and middle. There is hardly any Ragveil in the middle section, so it's best to take a route that allows you to cover all of the eastern and western sections of the zone.

Terocone, like Ragveil, is common but only present in Terokkar Forest and the forests in Shadowmoon Valley. There is no Terocone present in the Bone Waste, so do not search that area. The best route to take is to circle the Bone Wastes and then search the northern part of the zone. You can also just fly over the two forests in Shadowmoon Valley and farm Terocone while also looking for Nightmare Vine.

Nightmare Vine is only present in Shadowmoon Valley and is slightly uncommon. The best route to take is to fly from the northwestern corner down to the southern tip and then towards the Black Temple. Make sure to search all of the Black Temple, as there are lots of locations for it there. Then fly virtually due west from the temple back to the northwestern corner. If this route becomes unrewarding, then search the inside of the zone. Most of the spawn points for Nightmare Vine are contained around the edges of the zone.

Netherbloom is fairly easy to farm, but can only be farmed in Netherstorm. The easiest route to take is to go around the edges of each island inside Netherstorm. There are 6 islands total that contain Netherbloom. The only one that does not is the small island in the very northwestern corner of the zone.

If you follow these routes for each type of herb, you should have no problem getting multiple stacks per hour, assuming competition is not very high. The stacks of herbs sell for various prices, ranging from 15 gold a stack per Felweed and over 50 gold a stack for Nightmare Vine, but that generally reflects the amount of time it takes to obtain them, as Felweed can be farmed quickly and Nightmare Vine slowly.

When you follow these routes across the world of warcraft, you'll make cash very quickly.