Marking packets as dscp cs3 and cs4

I’m needing to do VoIP priority for some hosted phones (Astra 6757i) and I’m needing to mark them by using DSCP cs3 and cs4. I’m fairly new to QoS so I’m not quite sure where I need to do this at.. I know it’s in /firewall/mangle but as far as the option for setting it to cs3 and cs4 I’m not sure. Any help is appreciated. Thanks!

You will identify the traffic by whatever method makes the most sense (IP address, ect) and then you will choose action=change DSCP (TOS).