Gather fresh rosemary sprigs, about two to three large handfuls, and wash them thoroughly to remove any dirt or debris.
Allow the rosemary to dry completely by allowing it to air dry for at least few hours or by gently patting it dry with a clean towel. Any moisture remaining on the sprigs will cause it to mold and spoil the oil.
Once your rosemary is dry, it's time to infuse it into a carrier oil. The most commonly used carrier oil for this purpose is olive oil, but you can also experiment with other oils such as coconut or jojoba oil.
Remove the rosemary leaves from the sprigs and chop them coarsely to release their aromatic oils.
In a small saucepan, heat the carrier oil over low heat until it becomes warm but not boiling. Add the chopped rosemary leaves to the warm oil and gently stir to ensure they are fully immersed.
Allow the mixture to simmer on low heat for about 2 hours, stirring occasionally to prevent sticking.
Remove the saucepan from the heat and let the rosemary oil cool completely.
Once cooled, strain the oil through a fine-mesh sieve or cheesecloth into a clean glass jar or bottle.
Discard the leftover rosemary leaves and seal the container tightly.
Store in a dark place.