New Module That Changes PWM of the servo

Hi all , I am trying to create a new module that runs only when i call it. Its kinda a like package delivery initially drone is loaded with payload. When the module runs it changes the servos pwm and package drops. I don’t need automation so i am not putting it into the navigation part. I am using px4 version 1.12. I am aware of that there exist a package delivery module in 1.14 but i am not using griper or smth like that. Also i am realy new at px4 so far i learned how to create custom send and receive mavlink uORB messages. But in practice i didn’t write any new module similiar to this. Is there any documentation i can read that explains how threads work run part etc. Can I use mav_cmd_do_servo? Any suggestions or any module thats similar to what i’ve explained. Thank you in advance.