Diagram shows maximum handicap of 4 for 9x9 Go board with handicap stones numbered.  To setup a handicap of "n" place on the board the stone[s] numbered "n" and all stones with the lower numbers.

Since 9x9 Go board is used for training only, its handicap system is not one that is generally accepted.  Some players would argue, for example, that placing more than one handicap stone on the board that small would be equivalent to giving victory to Black even before the start of the game.  If this argument is accepted, 9x9 board is left with the two step handicap: weaker player getting Black without giving Komi as the first step, and Black placing it's handicap stone right into the center of the board as the second.  When players have close strengths this would be a more reasonable system of using handicap for 9x9 Go board.