Hard patch a 1/4 inch cable out of the 1/4 inch Main Left or Right into Aux In A or Aux In B. If you're using the 1/4 inch main outs instead of the XLR outs, then assign the mics you need to a Sub Out and hard patch a 1/4 inch cable out of that Sub out into Aux In A or Aux In B. Don't assign an output for the Aux In you are using.
Hit the Mix button twice on the Aux Send you're using and you'll be able to adjust the first few knobs. Look for the meter labeled Aux in A or Aux in B and turn the knob up on the Aux In you're using. From here, you can use the Aux In knob to feed more of the Main Mix into your Aux Send.
Keep in mind that if you use the Main out into an Aux In, you won't get any audio going into the Aux send if the Main Mix is down so it might be better to use a Sub out instead. Also, using a Sub out will give you the ability to delay your Aux Send, if you need to.