Your master (stereo) outs will go to your main system (mains), the Aux outs are the same signal usually pre fader. Just take your master Aux out and send it into your monitors. Then the Aux knob on each channel will determine the mix that is sent to the main Aux out.

If you use Pre fader then your monitor mix wont change until you change an Aux knob on an individual channel. If you don't and someone runs the channel fader up higher you can experience a change in the mix and possibly feedback. You want to think of your monitor send as a completely different mixer that YOU have total control over. Do what ever you want to the mains, it wont effect you. That way you stay happy on stage with your monitor mix.

That board, with the switch on the Aux buss, will let you have 2 monitor mixes using the Mon output and Aux1 output. They can both be Pre Fader or 1 Pre and 1 Post. You choose. The FX is a separate Send.