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
Inks
|
Pigments
|
Herbs
|
| Moonglow Ink Ivory Ink |
Alabaster Pigment |
Earthroot, Peacebloom, Silverleaf |
| Midnight Ink Hunter's Ink |
Dusky Pigment Verdant Pigment |
Briarthorn, Mageroyal, Swiftthistle, Stranglekelp, Bruiseweed |
| Lion's Ink Dawnstar Ink |
Golden Pigment Burnt Pigment |
Kingsblood, Wild Steelbloom, Grave Moss, Liferoot |
| Jadefire Ink Royal Ink |
Emerald Pigment Indigo Pigment |
Dragon's Teeth, Khadgar's Whisker, Goldthorn, Fadeleaf |
| Celestial Ink Fiery Ink |
Violet Pigment Ruby Pigment |
Blindweed, Firebloom, Ghost Mushroom, Gromsblood, Purple Lotus, Sungrass, Arthas' Tears |
| Shimmering Ink Ink of the Sky |
Silvery Pigment Sapphire Pigment |
Icecap, Sorrowmoss, Mountain Silversage, Dreamfoil, Golden Sansam |
Outland
Northrend
Cataclysm
Pandaria
Draenor
Legion
Kul Tiras / Zandalar
 Ultramarine Ink,  Crimson Ink,  Viridescent Ink |
 Ultramarine Pigment,  Crimson Pigment,  Viridescent Pigment |
 Anchor Weed,  Akunda's Bite,  Riverbud,  Siren's Pollen,  Star Moss,  Winter's Kiss,  Sea Stalk |
 Maroon Ink |
 Maroon Pigment |
 Zin'anthid |
Shadowlands
Warbinder's Ink
Cerulean Pigment
Fireweed, 
Frostweed, 
Gorgrond Flytrap, 
Nagrand Arrowbloom, 
Starflower, 
Talador Orchid
Astral Glory, 
Yseralline Seed, 
Aethril, 
Dreamleaf, 
Felwort, 
Fjarnskaggl, 
Foxflower, 
Starlight Rose
Sallow Pigment
Roseate Pigment
Luminous Ink, 
Tranquil Ink
Luminous Pigment, 
Umbral Pigment, 
Tranquil Pigment
Nightshade, 
Death Blossom, 
Marrowroot, 
Rising Glory, 
Widowbloom, 
Vigil's Torch