Use water from an electric kettle or water heated on the stovetop in a tea kettle. Heat the water until it starts making noise in the pot, or feels warm if you wave a hand over the spout - around 110°F to 120°F. Empty the kettle into a heat-resistant container with a pour spout, and add cool water until you can dunk a hand in it without pain. Place your bottle in the sink, grip it near the top, and fill the water bottle two-thirds full. Position the bottle so it’s slightly angled, and lower it to the point where water is visible at the top to remove any trapped air. Close the bottle and feel the outside to ensure it’s not dangerously hot.