If it works like other aircraft with VNAV and no auto-throttle it will be something like this.
You set a VS to initiate a decent at a certain point. At that point the plane begins to decent at that VS. Then you need to set the throttle to match the desired speed.
or
You set a SPEED (IAS,FLC) to initiate a decent at a certain point. At that point the plane will hold that speed and you will controll VS with the throttle.
or
I could be completely wrong as I have not tried it in the CRJ, caveat emptor.