Place the loin, fat side down on a large cutting board
With a large sharp knife, make a long cut, an inch from the edge and stopping at about half an inch before reaching the bottom of the loin. Continue cutting the loin with the knife angled parrallel to the cutting board and unroll the meat while you cut through it. You will be done cutting when you have a flat meat surface unrolled.
If you want a flatter surface, you can use meat pounder to get the loin even everywhere.
Spread the stuffing on the flat meat surface, leaving around 1 inch margin on the edges.
Start rolling back the loin, starting from the non-fat long edge. When you are done, the fat of the loin shoud be the top of the roll.
Tie with kitchen twine, leaving a space of about 1-2 inches between each knots.