Amarillo and nyc- Yea, I just slit it down the middle, starting at the top, about 1/4" from the end, stopping about the same at the bottom.
Use a really sharp, finely-pointed knife. In the 2nd pic, the boning knife in the bottom left is what I used.
Make shallow cuts the full-length of the loin, using just the point of the blade. Repeat this gradually getting deeper each cut. As you work, use your thumb to open the cut, running it up and down spreading the slit apart.
Once you get halfway down, you can kinda angle the blade into each side of the meat, opening each side as you also cut down as well.
Then plastic wrap over it and gently pound it out to desired thickness.
Here's a video I found that gives you a pretty good idea of how it's done.
It's really pretty easy, just go slowly at first and you'll get the hang of it in no time.