It's not as easy as you would think.
My first attempt to zero a pair of shoes were my beloved Montrail Mountain Masochists. I'd been running in my Vibrams for a couple of months, as well as hiking and wearing them to Home Depot. I decided, since I was planning on racing in them, to go back out on a run in the MMMs.
It was a pain. The heel kept interfering with my stride; I was regularly jamming it into the ground. So when I got home I realized that I was not going to be wearing the MMMs any more, although they were one of the better sneakers I'd ever owned. I followed Anton Krupicka's example, bought a bread knife at Walmart (so as to not use one of my wife's bread knives), and attempted surgery.
It wound up like Tim's attempt. I severed the heel from the upper. Whoops.
My second attempt involved a rotary table saw and the L.L. Bean Katahdin Iron Works Engineer Boot. This is one of the few non-military Munson last boots available, and is amazingly comfortable to wear, aside from the heel.
Let's just say the patient didn't survive, and I was lucky to retain all my fingers.
The next pair of shoes I wanted to modify went to the cobbler, who explained, in passing, that severe cuts were the norm when first working as a cobbler. OK, then.