When you create your granny squares there is ultimately going to be some curled up edges or distortion and so if you want them all to be the same size you need to block them into shape.
Blocking means easing the finished crochet into shape by pinning to either a fabric covered board and using an iron to steam the piece, useful for wool, cotton or linen yarns or for acrylic and other synthetic fibres a foam board is used and the piece is sprayed with cold water and left to dry.
The cold water method is what I have used for my granny squares as the yarn is 100% acrylic.
The foam board I have used is shaped like a jigsaw piece and other pieces can be added to create a larger area. I have also used some T-pins which are easier to push in than dress makers pins.
I knew that I wanted my granny squares to be 6 inches square and luckily my foam is already marked out in inches.
I pinned out my first square starting at the corners, then the centres etc. Use as many pins as necessary to get the square even.
I then sprayed the square with cold water and patted it down to help the water penetrate the fibres.
You want the granny square evenly moist but not soaked!