To demonstrate that matter is conserved during a physical change, the most appropriate procedure is:
"Find the mass of a cup of water. Boil the water. Find the mass of the water that remains."
This procedure focuses on a physical change (the boiling of water) and would allow you to compare the mass before and after the change. Despite the water changing state from liquid to vapor, if conducted in a closed system, the mass should remain constant, demonstrating the conservation of matter.