Ok… yeah forget my first attempt… look at the manual, page 40 gives some nice layouts. The monitors do hook up to the mixer. the mixer sends the signal out to the cpu then back to the mixer out the master bus to the speakers, you can then play other instruments on the other mixer channels and blend to taste and record at same time by arming the corresponding channel in cubase.
manual; Soundcraft - Professional Audio Mixers. sounds like you may have some head scratching to do though 
This is really straight forward, and signal flow is done in the mixer…nice little mixer by the way.