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Fill a clean glass, lidded jar with your herb of choice. The herb should be packed loosely in the jar. Pickling or preserving jars work very well for this. Fill the jar with your chosen base oil, making sure to cover all the plant with the oil. Cover the jar tightly. Cover the bottom of a baking pan with enough water to cover the bottom half of the jar. Then, place the jar with the herbs and oil into the pan and place in your oven. Turn the oven on with the lowest heat possible, usually 200o F, and let it cook for five to six hours. You can also preheat the oven to the lowest possible temperature, place the pan with the herb and oils in the oven and turn off the oven. If you do this, you will have to repeat the process by re-heating the oven over the five to six-hour period. The goal is to have a steady temperature as possible. Use an oven thermometer to help you keep track. Check every hour or so to be certain the oil is not bubbling or smoking. If it is, carefully removes the pan and let the oven cool down a bit. Once it cools, put the jar back into the oven. After five or six hours, carefully remove the pan from the oven. Allow the jar to cool down by letting it sit. When it is cool, pour the oil through a cheesecloth into a new clean glass, lidded jar. This separates the herbs from the oil. Store the oil in a cool, dry, dark place.
Place the oil and herbs in the jar. Put the jar in the oven. Remove the jar.