This was how I learned the dummy (and the guy I learned it from).

Train the waist.  Don’t beat the crap out of it – its not for “hardening” the arms (whatever that means).  Chase center.  Develop structure.  Move as one.  Develop your angled approach.  This will tighten up in Chi Sao/Gwoh Sao.  Tighter and tighter angles from a 45 degree change on this stationary training tool until its just a few percent – so you can make yourself correct and make them wrong, as Sifu Lam says.