In keymap_common.h :

#define KEYMAP( \
    K00, K01, K02, K03, \
    K10, K11, K12, \
    K20, K21, K22, K23, \
    K30, K31, K32, \
    K40, K41, K42, \
) { \
    { KC_##K00, KC_##K01, KC_##K02, KC_##K03 }, \
    { KC_##K10, KC_##K11, KC_##K12, KC_NO}, \
    { KC_##K20, KC_##K21, KC_##K22, KC_##K23 }, \
    { KC_##K30, KC_##K31, KC_##K32, KC_NO }, \
    { KC_##K40, KC_NO, KC_##K41, KC_##K42}  \
}
#endif

-----------------------
In config.h :
/* key matrix size */
#define MATRIX_ROWS 5
#define MATRIX_COLS 4

-----------------------
In keymap_poker.h :

#include "keymap_common.h"

const uint8_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
           KEYMAP(KC_ESCAPE,KC_KP_SLASH,KC_KP_ASTERISK,KC_KP_MINUS, \
           KC_KP_7,KC_KP_8,KC_KP_9, \
           KC_KP_4,KC_KP_5,KC_KP_6,KC_KP_PLUS, \
           KC_KP_1,KC_KP_2,KC_KP_3, \
           KC_KP_ENTER,KC_KP_DOT,KC_KP_0),
};

const uint16_t PROGMEM fn_actions[] = {

};