Unity 2D Mobile Game Drawing Mechanic

My team and I are creating a simple mobile game that utilizes fast passed action using touch controls.

In this game we want to implement a spell casting feature similar to that of Lost Magic for the DS. For instance...https://www.youtube.com/watch?v=-JYTGfagvWY

The user will open an area where they can use their finger to draw a shape of some sort.

Looking online, I found these links that don't quite match what I'm looking for.

answers.unity3d.com/questions/318602/draw-line-in-2d-space-using-linerender.html answers.unity3d.com/questions/601640/correct-way-to-draw-on-screen-for-2d-game.html www.everyday3d.com/blog/index.php/2010/03/15/3-ways-to-draw-3d-lines-in-unity3d/

What is the best way to draw a simple shape and have it recognized by unity in order to cast a spell for example?

Any help is appreciated! (I need to get this done by the end of April :D)