Measuring

You keep saying add 40 grams flour and 40 grams water. How do you measure this? You can't put this on a scale. It will go all over the place with nothing to hold it. Do you have to weigh the vessel first each time? How do you add to a vessel after the first time? How do you know how much to remove each time before feeding it? Do you have to measure the take off so you can add the new addition? Does it have to be equal?