qmk_firmware/keyboards/lets_split/keymaps/default/config.h

#ifndef CONFIG_USER_H
#define CONFIG_USER_H

#include "../../config.h"

/* Use I2C or Serial, not both */

#define USE_SERIAL
// #define USE_I2C

/* Select hand configuration */

#define MASTER_LEFT
// #define _MASTER_RIGHT
// #define EE_HANDS

#endif