One of the rarer playstyles in Hearthstone is the 'mill' archetype - working towards a win condition of burning your opponent's cards and killing them with fatigue damage. Druid is still the class best poised to manage a mill deck - read on to learn a little bit about how to make it work.

Deck Composition

To make an effective Mill Druid deck, most people will rely on a fairly consistent set of required cards:

2x Naturalize
2x Wrath
2x Youthful Brewmaster
2x Coldlight Oracle
1x Goblin Sapper
2x Grove Tender
2x Keeper of the Grove
2x Swipe
2x Clockwork Giant

Variants will include certain options such as:

Innervate
Dancing Swords
(Card #1890)
King Mukla
Mechanical Yeti
Loatheb
Starfall
Ancient of Lore
Troggzor the Earthinator
Tree of Life

Some players will also add a variety of healing solutions to extend the duration of the game in their favour.
 

Playstyle, Tips, and Notes

Mill plays unlike anything else you've used in the past. You still value certain core concepts, like a degree of board control, but your primary objective is to force your opponent past their hand limit to burn cards and to accelerate the point at which they begin taking fatigue damage.

As a general rule, milling does not work on aggro decks - you might be able to burn some of their cards, but you will give them as many (or more) answers than you destroy. Irrespective of your opponent's playstyle, you have to go into each game with a plan that is centered around forcing card draw at inopportune times while establishing your own board presence and having adequate removal to react to the options your forced draw will invariably give them.
 

Mulligans

What to mulligan for will vary slightly with individual decklists, but the general rule of thumb is to look for Grove Tender, Coldlight Oracle, and/or Wrath. A lot of your milling cards also act as a draw for you, which can help you dig for further answers to your opponent's deck.
 

Combos and Counters

Lots of your plays will set up Goblin Sapper and Clockwork Giant; you also have one explicit combo:

Coldlight Oracle + Youthful Brewmaster