There is actually an example launch in the PX4 repo, here: https://github.com/PX4/Firmware/blob/master/launch/mavros.launch You would need to change the fcu_url and gcs_url to suit your needs, but otherwise that launch file would launch mavros properly. As you can see there are two extras files of concern, pluginlists_yaml and config_yaml, both those files are in mavros' launch directory, and if you want to modify them, say disable or enable some plugins you can either change those files or point the arg to your own yaml config file

EDIT: if you have mavros_extras installed just add the appropriate plugins to the pluginlists_yaml