How to get two channels recording on Yamaha MG12XU

Help me please. I have set up cubase elements 8 with my yamaha MG12XU and I can’t seem to be able to record two instruments at the same time on channel 1 and two?
They seem to record across both channels. What am I doing wrong?
Also I can’t work out how to have playback through the headphones on the MG12XU and record one channel whilst the others are playing in my ear?
Can someone give me an idiots guide to set up this basic function as Im tearing my hair out and otherwise will have to go back to garage band and try that.
thanks x