A miller is a dwarf who mills plants at either a quern or millstone, placing the resulting goods into bags. Milling produces one unit of dye, flour or sugar per item milled. Milled items have no extra quality associated with them.
A quern must be operated by the dwarf himself, while the millstone uses outside mechanical power (either a water wheel or a windmill).
To mill a plant you need the plant to be milled and a bag. Milling does preserve plant seeds.
There is no easy way to determine which plants a dwarf will mill first, you will have to forbid plants you don't want to be milled.
Milling plants is always done at 1:1 ratio; item number stays the same, but value is increased. All plants except whip vines mill to 10× plant value; whip vines are 25% better.
List of millable plants
Plants that make powders (need to be cooked)
Plants that make dyes