Prepare the plant material. For herbs, use only the leaves and flowers and chop roughly. For spices, use whole spices and crush in a mortar and pestle. For flowers, use only the petals. For kitchen flavorings such as garlic and citrus peels, use fresh materials in season.
Measure the plant materials. Typical ratios per one cup of carrier oil: 1/4 ounce herbs; 2 tbsp. crushed spices; 1/4 to 1/2 oz. flower petals; 6 crushed garlic cloves; or 2 tbsp. chopped citrus peel.
Place the carrier oil in the crockpot. Use neutral oil such as jojoba, safflower or corn oil for herbs and spices, or extra virgin olive oil or nut oil for kitchen flavorings.
Place the plant materials directly into the oil or wrap in several layers of cheesecloth.
Cook on low for six to eight hours. If the fragrance is not strong enough, cook for a couple hours more, not to exceed 12 hours. If the fragrance is still not strong enough, remove the spent plant materials and add fresh in the same ratio, taking care to not overcook.
Strain through a fine mesh strainer or remove the cheesecloth bundle. Store in brown bottles in a cool, dark place and use within six months.