Hello Community, i’m currently developing a project where im coding the control law, the quadcopter im using is a coex clover 4.2 with a px4 card, my doubts are related to the way I can create my own control
law, what codes should I check or modify in the firmware to do that, thank you in advance, grettings
How about starting with this? 
Thank you for your help, I’ll check it out, I have another question, the way we upload the Firmware to the drone is by QGroundControl, the firmware is a single file (.px4), how can I go from the repository you send me (with my adjustments on the folders) to that single file that we upload to the drone?, Thanks in advance, greetings.
You need to build your own px4 file. Depending on the board you have. For example, if you have Pixhawk 4, you can run the command make px4_fmu-v5 or for Pixracer make px4_fmu-v4.
Side notes: it will be easiest for you if you can run Ubuntu and prepare the environment so you can then run this setup script, which will install all that is needed for a successful firmware build.
I’m gonna add this entry from the docs to it, where you can find a list of various flight controllers and their corresponding make command.
For additional information on the toolchain setup, you can take a look at the documentation as well. Here you can find the setup for Ubuntu Linux, including the mentioned setup script.
I appreciate it very much, I will work on it, thank you