how to record two instrumentals on my yamaha mg12xu

Please can someone help me. I have loaded Cubase elements 8 with my new yamaha MG12XU and I can’t work out how to record two channels at once. They both seem to record across both channels. Also how can I playback without this feedback.
Can someone give me an idiots guide to set up two channels for instruments - a guitar and electronic drums.
thank you