Boil water in a jug/kettle and let it cool down to about 80°C/175°F (about 20 minutes or so).
Place rosemary in a large enough mason jar, a bowl or a saucepan and add water.
Steep rosemary for at least 5 or up to 12 hours (maybe even a few hours longer, or simply overnight, if you like).
Strain rosemary infusion. You can pour the infusion into a spray bottle or a jar.