Side view. Here you can see the picanha. It is the lower muscle with the fat cap on bottom.
As you can see, it's very easy to separate the two muscles.
Follow with your knife down the natural seam.
This is how it looks when you get to the end. Their is a thick layer of fat at the end where the two muscles are still connected. That is where you cut to separate the picanha from the rest of the sirloin butt.
Bottom view. Leave the fat cap on.
Trim off the thick layer of fat that connected the butt and picanha.
Here it is cleaned up after removing the silver skin and excess fat.
You want your slices at least 1.5" in thickness, sliced across the grain.
Here you have it, ready to hit the grill!