Players take an extra knight, and put this aside the board. Once during the game, a player may put this extra knight on any free square on the board, instead of making a normal move. After this, the knight moves as a usual knight.
All other rules are as in the orthodox chess game.
Note that there are no restrictions to the square where the knight is placed, as long as this square is empty. Hence, it is allowed to put the knight on such a square, that check is given, mate is given, or, when a player is in check, between the checking piece and the king to remove the check.
But in Pocket Chess, a player can SECRETLY choose either to start with an extra Knight or an extra Bishop.