How to play nine men's morris (see also this video)
There are three phases in the game (one set-up phase and two move phases). In every phase, you can capture if you make three in a row (horizontal or vertical), this is called a 'mill'. If you form a mill, you must capture an opposing piece of your choice (except when this piece is part of a mill and your opponent has other pieces which are not part of a mill).
In the first part of the game, the players each place a piece in turns on an intersection of a horizontal and a vertical line. This phase ends when every player has placed his 9 pieces
When both players placed all their pieces on the board, the second phase starts. When it is your turn, you must move one of your pieces to an adjacent free place (along a line). If you form a mill, you must remove an opposing stone from the board.
The third phase starts when a player has only 3 pieces left. That player is allowed to move his pieces to any place on the board instead of just an adjacent place. When both players have 3 pieces left, both of them may do so.
You win the game by either capturing 7 opposing pieces (so the opponent has just 2 pieces left) or if your opponent has no legal move.