How to create new message type

[irsyadtc]

Hi, I would like to know how to create a new mesage type (e.g. sensor_msgs/NavSatStatus & sensor_msgs/NavSatFix).

So far, I know that the messages library is located at /src/ros2/ and I found comments in several .hpp files that they are generated via tool gen.

/*! 
 * @file Imu.hpp
 * This header file contains the declaration of the described types in the IDL file.
 *
 * This file was generated by the tool gen.
 */

I wish someone can show me how.
Thanks

PS: Yes, I'm aware that Micro-ROS is available but since my machine is using Ubuntu 18 & ROS2 Dashing, I stick with this library for compatibility.