An important thing to note in squash and stretch is that the volume of the object is retained i.e. it doesn’t get changed even if the object is deformed.
A water balloon in the below example is capable of stretching and squashing but its volume remains constant. The water inside it neither increases nor reduces while the stretch and squash happen.
For practicing stretch and squash it’s better to try animating a bouncing ball and apply the principle to understand how stretch and squash work.