In surveying, one of the fundamental principles is to work from whole to part.
Suppose you have to draw a map of your house. You start drawing a room. Then another and so on. If you did some mistakes in drawing any room, then every room will be wrongly placed. It is more likely that you will cross the boundaries of your house. Your mistake will have an accumulating effect.
To avoid this, we should first draw external boundaries. Then proceed towards internal details. By doing so, the mistake has a local effect and can be corrected easily.
I am a civil engineer. I use this principle in my general life too.
So, what is the mental model?
A mental model is an individual’s idea of how the world works.
The quality of our thinking is proportional to the models in our mind.