Take a TRS patch cable and connect one end to the FX OUT on the back of the mixer and the other end to the line input on one of the channels.
Next turn the AUX knob on the AUX send where your monitors are on. This will apply whatever FX you have to the monitors!
EXTRA NOTES: Make sure that you DO NOT apply FX using the AUX 3 FX knob or this will create a feedback loop. You can either bus this channel to the mains or leave this channel solely for the monitor FX send depending on your preference.