Doughing in is the name for adding the grist to the strike water. You measure the temperature of your grain, you know the weight of your grain, and you know the weight of the strike water. It's a fairly simple calculation to determine what temperature your strike water should be in order to achieve your desired mash temperature. I always add the strike water to the mash tun a few degrees hotter than that, and wait for the temperature to stabilize and cool to strike temperature. Then stir in the grain.