To make essential oil using cold infusion, place dried leaves and stems in a jar and cover them with oil. Leave the jar in a well-lit environment for two months, and then strain off the resulting oil. To make a hot infusion, simmer the ingredients for three hours and then strain them.
Different parts of different plants are suited to making essential oils. Harvest flowers and leaves in the morning, and harvest sap and resin in the autumn. Harvest fruits and seeds just before they ripen, and wait until the plant has wilted before harvesting roots and bulbs.
Clean dirt and bugs off of the plant material before hanging it up to dry in a warm, dry room. Tie a paper bag around the ends of bunches of flowers and twigs.
For cold infusion, fill a canning jar with plant material, and pour oil around it to completely fill the jar. Coconut oil, olive oil and jojoba oil are some oils that work well for this purpose, provided the oil is food-grade. Fasten the jar and leave it in a sunny room, but out of direct sunlight. Wait for two months before straining the mixture with cheesecloth, and then store the oil in a cool, dry place.
Essential oils that are available commercially may be expressed, or squeezed, from sources such as lemons. Other plant oils are steamed out, or extracted using solvents.