Fill a small pan with MORE THAN enough water than your ice tray wil hold (some water will be lost to evaporation)
Bring to a nice medium gentle boil for 5 Min.
Shut off the heat and let it set until room temperature.
Repeat and bring to a Boil for 5 Min again.
Shut off the heat and let it sit still until room temperature.
Carefully fill your ice tray with the cool water (as little disturbance of the watter as possible)