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. Separated by level from lowest Inscription skill to highest.
You can find herb farming guides for most herbs on my website.