This set up is to make the score line on the the first can.
1. Clamp a razor to your workbench 1.-1/4" above the surface of the bench. 2. With the can upright, slowly rotate the can against the razor with only slight pressure. 3. Once you have made several passes around move to the next step 4. Use the clamped razor to carefully puncture the can along the score.
Be ready with a sink or bucket or be able to get outside quickly. (things get messy here)
5. Carefully work your thumb in to the puncture as shown in the picture, and slowly work your way around the can. The can should separate along the score line if it was done properly.