Mixing plaster to the correct consistency is crucial in obtaining good results. Some beginners have a tendency to mix the plaster up too thick which can make it harder to keep the plaster flat and to the correct thickness. It does want to be spread very thinly and keeping it flat is crucial - therefore a nice thin mix will help to ensure good results. The only downside is that the wetter the plaster is - the harder it is to control, but this gets easier. To help gauge the consistency this short video purposely shows a mix that at first is too thick.