How do I remove *part* of a *non-square* backsplash?

We're going to put in an over-the-range microwave but we need to remove some backsplash to have vertical room. From what I can tell, the backsplash isn't ...even (?) though. You'll see in the pictures.

I've done some reading and watching online and I get the gist: use a hammer and spackle knife to get behind the tile and pull it off the wall. But what I don't get is how to get the tile on our backsplash to come off evenly like that square I drew on the picture. Do I use something to cut it?

Thank you.

Submitted August 30, 2015 at 07:11PM by zipzoomerooer http://ift.tt/1O1nXyg