Starting at the breastbone, cut each breast half completely away from the rib cage. You should have 2 boneless filets of breast meat.
Spread each breast filet flat on a cutting board. I am right handed, so I place my left palm flat on the breast filet, and use a sharp knife to slice the filet *almost* entirely in half (making the breast filet thinner) by slicing the meat evenly between the cutting board and my hand. The idea is for the breast filet to look like a "butterfy" with spread wings when you are done to facilitate wrapping around the filling and faster cooking time.
Here is a crude diagram:
Hand
Meat ---
--
Knife >>> --
--
Meat ---
Cutting board
I can certainly demonstrate this much better than I can explain it!