Controlling Rover using mavros

Hello everyone,
I’m trying to build a “Skid Steering” rover using (navio+raspi3+ardupilot). As a first step i wanna control the rover using cmd_vel and the waypoint command.
how can i configure the OUTPUT i want to use for the PWM ?
As a new user of ros i don’t know how to proceed to write the scripts. someone can help?