First, visit any trainer in the main cities of old Azeroth - just ask a guard, and learn Apprentice Herbalism.

For the first 70 points, you can go to any starter zone, there is not much difference between them. I recommend the two following zones, because they are really close to the zones that I recommend for the next part of the Herbalism guide.

You will gather the following herbs: [Peacebloom], [Silverleaf], [Earthroot]

Tirisfal Glades

Elwynn Forest

70 - 150 Herbalism

You will gather the following herbs: [Mageroyal], [Briarthorn], [Bruiseweed]

Learn Herbalism Journeyman then head to Hillsbard Foothills because that is the best zone for this part of the Herbalism leveling guide. It's a horde only zone, this means it might be a bit harder to reach it for alliance players, and for low level players I recommend Darkshore and then at 115 go to Ashenvale, or Westfall and then at 125 go to Northern Stranglethorn. The reason I recommend Hillsbard is because you can level up to 150 in one zone.

Hillsbard Foothills

Between 70-125, just follow the yellow lines. When you reach 125, you can enter the orange circle and gather Frozen Herbs.

150 - 230 Herbalism

You will gather the following herbs: [Kingsblood], [Fadeleaf], [Khadgar's Whisker],[Goldthorn](Feralas only)

Visit your trainer and learn Herbalism Expert. For the first 10 points until you reach 160, you won't be able to gather Khadgar's Whisker, and for the last 10-20 points, all of the herbs will be green, but it's still the fastest way if you to stay in these zones.

You will have to visit your trainer again when you reach 200.

Feralas

Western Plaguelands

230 - 285 Herbalism

You will gather the following herb: [Sungrass]

In both of the zones below the most common herb will be [Sungrass]. I recommend Thousand Needles, because in Eastern Plaguelands there are a lot of [Khadgar's Whisker] which will be green to you and grey for the last few points. I also like the easy layout of Thousand Needles, you just go in a straight line and you can't miss the herbs, but don't get me wrong, Eastern Plaguelands is a really great alternative.

Thousand Needles

If you don't have a flying mount, then when you reach the end of the yellow line, you should turn back and run back to the starting point of the yellow line. You will also have to swim to Twilight Bulwark or Twilight Withering, else you can't really get to the place where the herbs are. (you can see these zones if you click on the map below)

Eastern Plaguelands

285 - 325 Herbalism

You will gather the following herbs:

Swamp of Sorrows: [Golden Sansam], [Sorrowmoss]

Felwood: [Gromsblood], [Purple Lotus], [Dreamfoil], [Golden Sansam]

Visit your trainer and learn Herbalism Master.

Swamp of Sorrows

The whole zone is filled with herbs. You can find them everywhere, there is no farming route for this map.

Felwood

325 - 350 Herbalism

You will gather the following herbs: [Felweed], [Dreaming Glory], [Terocone]

The orange circles means that you should fly around that zone, because the herbs are spread all over inside them.

Terrokar Forest

350 - 400 Herbalism

Learn Herbalism Grand Master. In Howling Fjord, you will mostly find [Goldclover], but there will be some [Tiger Lily] near the rivers and lakes, you won't be able to gather these until you reach 375 in Herbalism.

Howling Fjord

400 - 425 Herbalism

Sholazar Basin is filled with Herbs. The most common herb is [Adder's Tongue], but you can also find some[Goldclover] and [Tiger Lily]. If you don't really care about selling the Herbs, you should stay in Sholazar Basin until you reach 450. All it takes is just one full circuit in the zone, assuming no one else is farming there.

Sholazar Basin

425 - 475 Herbalism

Visit your trainer and learn Herbalism Illustrious.

You will gather [Cinderbloom].

Mount Hyal

475 - 500 Herbalism

You should go to Deepholm and gather [Heartblossom] or you can stay in Mount Hyjal until you reach 500.

500 - 525 Herbalism

You will gather [Whiptail].

Uldum

I hope you liked this Herbalism leveling guide! Gz to 525!