Laser PWM and M codes

Hi i'm new to the world of linuxcnc and G codes. I have succesfully built and tested a CNC gantry router table. A friend brought me a laser to install on my machine. I have been looking at the forum on how to implement this and found it very difficult to do. I have found two ways to go about this problem.

The first one is to use the spindle's PWM and implement M3,M4 and M5 codes. The second one creates a pwmgen signal just for the laser and uses M67 and M68 codes.
I read a lot about these two but i have very little knowledge about implementing M codes. I did read all the files in the Linux M codes.

My questions is what signal would be the best to use and where do i learn how to modify the G code to implement the M codes ?
 

I got the laser PWM hooked to the spindle's output and i used M3 sxxx on negative Z et M5 on positive Z movements. it does work but it takes a while to do manually. Is there an easier way to edit the G code? Post processor or a Python program maybe used to match the Zès movement to the PWM?

I will look at it, thanks Tommy.