Oddjob007,
There are actually two ways to go with the hairspray and the baking powder.
The first is to spray the hairspray first from above and the dust with the baking powder immediately before the spray hardens off. This is the quickest way but any extra "snow" which falls will also need to be sprayed to keep it in place too.
The second is spray with the PVA / water mix first then the baking powder when the PVA mix begins to dry somewhat then finally spry the hairspray to keep it all in place.
This works well too but it is a little more time consuming.
HTH , Cheers , Pete.