Seems simple enough:
const byte ROWS = 4; // Four rows
const byte COLS = 3; // columns
// Define the Keymap
char keys[ROWS][COLS] = {
{'1','2','3'},
{'4','5','6'},
{'7','8','9'},
{'*','0','#'}
};
// Connect keypad ROW0, ROW1, ROW2 and ROW3 to these Arduino pins.
byte rowPins[ROWS] = { 5, 4, 3, 2 };
// Connect keypad COL0, COL1 and COL2 to these Arduino pins.
byte colPins[COLS] = { 8, 7, 6 };