At the moment you can’t level inscription past 300. So revisit this guide and check if that changed :)

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You mil 5 herbs to 2 - 3 pigments*.
Each herb tier gives different pigments:

Alabaster Pigment: Bloodthistle / Peacebloom / Silverleaf / Earthroot / Mageroyal
Dusky Pigment: Briarthorn / Swiftthistle / Bruiseweed / Stranglekelp
Golden Pigment: Wild Steelbloom / Grave Moss / Kingsblood / Liferoot
Emerald Pigment: Fadeleaf / Goldthorn / Khadgar's Whiskers / Wintersbite
Violet Pigment: Firebloom / Purple Lotus / Arthas' Tears / Sungrass / Blindweed / Ghost Mushroom / Gromsblood
Silvery Pigment: Golden Sansam / Dreamfoil / Mountain Silversage / Plaguebloom / Icecap

To level Inscription to 300 you’ll need the following ammount of herbs:
(This list if for the worst case scenario where you would get every time 2 pigments from milling. You should need less materials.)

10 stacks of Alabaster-giving herbs (eg. 200x Peacbloom)
5 stacks of Dusky-giving herbs (eg. 100x Brianthorn)
5 stacks of Golden-giving herbs (eg. 100x Kingsblood)
6 stacks of Emerald-giving herbs (eg. 120x Goldthorn)
12 stacks of Violet-giving herbs (eg. 240x Blindweed)
19 stacks of Silvery-giving herbs (eg. 380x Dreamfoil)

In total you’ll need the following materials:

33 x Silverleaf
20 Silver Ink
81 Alabaster Pigment
39 Dusky Pigment
35 Golden Pigment
41 Emerald Pigment
95 Violet Pigment
150 Silvery Pigment
208 Light Parchment
76 Common Parchment
95 Heavy Parchment

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Inscription 0 - 125

2-30 - 33 x Silver Ink (1 x Silverleaf)
30-40 - 10 x Scroll of Intellect (2 x Silver Ink, 1 x Light Parchment)
40-50 - 10 x Moonglow Ink (1 x Alabaster Pigment)
50-75 - 60 x Scroll of Recall (1 x Moonglow Ink, 1 x Light Parchment)
75-85 - 11 x Bleached Vellum (1 x Moonglow Ink, 1 x Light Parchment)
85-100 - 17 x Glyph of Swipe (1 x Midnight Ink, 1 x Light Parchment)
100-120 - 22 x Treated Vallum (1 x Midnight Ink, 5 x Light Parchment)
120-125 - 6 x Glyph of Entangling Roots (1 x Lion's Ink, 1 x Common Parchment)

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Inscription 125 - 225

125-150 - 29 x Glyph of Healing Touch (1 x Lion's Ink, 1 x Common Parchment)
150-165 - 23 x Jadefire Ink (1 x Emerald Pigment)
165-175 - 12 x Glyph of Moonfire (1 x Jadefire Ink, 1 x Common Parchment)
175-200 - 29 x Glyph of Insect Swarm (1 x Jadefire Ink, 1 x Common Parchment)
200-210 - 10 x Celestial Ink (1 x Violet Pigment)
210-225 - 20 x Glyph of Rebirth (1 x Celestial Ink, 1 x Heavy Parchment)

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Inscription 225 - 300

225-250 - 25 x Glyph of Rip (3 x Celestial Ink, 1 x Heavy Parchment)
250-275 - 25 x Glyph of Starfire (3 x Shimmering Ink, 1 x Heavy Parchment)
275-300 - 25 x Glyph of Shred (3 x Shimmering Ink, 1 x Heavy Parchment)

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Inscription 300 - 375

The path to 375 is not implemented yet.

Credit for this guide goes out to http://wotlkwiki.info/index.php/Guide/Inscription