Sprayfo should be dissolved in water with a temperature of 45 to 55°C. If the water is too hot (over 65°C) the protein in the milk will be damaged.
Start with a portion of the water to add the milk powder, while stirring with either a whisk or a mixing machine.
Make sure the powder is dissolved well and then add water to get the desired amount. 
You can find the right dissolving ratio, based on product and species, on the product label.