How can i get more volume on my onstage monitors thru my mackie 2404 vlz3 mixer or what is the proper way to connect the monitors

Without more information of your speakers and amps we can't really help you. The Mackie board puts out line-level signals and that should drive any connected amp or powered speaker to the full capability. Make sure you are using BALANCED interconnect lines either XLR or TRS cables. Use of TS unbalanced cables costs you 6 DB and also noise pickup. Make sure ALL your gear, amps, speakers etc are powered from a SINGLE receptacle or power conditioning module. Normally you will use the AUX outputs of the mixer by way of TRS cables or balanced lines in a snake to power amplifiers onstage which then connect to the stage monitors. If you have powered monitors the returns of the aux to the stage go to the powered monitors. A heavy duty extension cord brings power back from thee stage power to the mixer.