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.