To get this you need to make certain shapes with your letters. The achievements in the menu show you which shapes you have and have not done yet.
After you play a snap the shape that is created is shown in a green outline.
So make sure you create the shape you want and it shows up with a green outline.
Wordament has created a picture guide of each shape being done in-game, and can be found athttp://www.wordament.com/get-in-shape-with-snap-attack/
I have made a quick mockup of the shapes so you can print/edit it to help keep track while in the game.This achievement does not carry over from the other version like some other achievements
Thanks to Xtowers for relaying Arsenic 17's info
Here is an example to get the 3x3 with the middle missing.
for the 3x3 square help here is an example: