Milling is an ability you get when you learn Inscription. Milling can be used to mill 5 herbs for 2 - 4 pigments, then you use the pigments to turn into inks. Below you can find a quick overview of which herbs converts to which pigments and which pigments converts to which inks.
You can find herb farming guides for most herbs on my website.
Don't forget that 1x
Umbral Ink can be traded for any Ink at Ink traders! You can level Inscription by using only Umbral Ink if it is cheaper than the lower level herbs.
Vanilla
Outland
| Ethereal Ink Darkflame Ink | Nether Pigment Ebon Pigment | Felweed, Dreaming Glory, Ragveil, Terocone, Ancient Lichen, Nightmare Vine, Netherbloom, Mana Thistle |
Northrend
| Ink of the Sea Snowfall Ink | Azure Pigment Icy Pigment | Goldclover, Tiger Lily, Deadnettle, Fire Leaf, Talandra's Rose, Adder's Tongue, Icethorn, Lichbloom |
Cataclysm
| Blackfallow Ink Inferno Ink | Ashen Pigment Burning Embers | Cinderbloom, Stormvine, Heartblossom, Azshara's Veil, Whiptail, Twilight Jasmine |
Pandaria
| Ink of Dreams Starlight Ink | Shadow Pigment Misty Pigment | Green Tea Leaf, Rain Poppy, Silkweed, Desecrated Herb, Snow Lily, Fool's Cap |