To make essential oils, oil need to be extracted from plants. This can be done through expression and distillation.

Expression:

Also referred as cold pressing, expression is a method used to extract oil from citrus fruits like lime, orange, tangerine, lemon, and bergamot. In the past, expression didn't require any sort of tool except for a sponge. After soaking the citrus rind or zest, it is repeatedly pressed against the sponge to absorb the oil. The sponge is then squeezed over a container to catch the oil, which is allowed to separate from its juices. The oil is siphoned off and bottled after a few hours.

A modern type of expression involves using a blender-like device that's equipped with spikes. Once the citrus zest, rind, or peel is placed in the device, it rotates, prods, and pricks the citrus until the oils are released. Oil are collected at the bottom of the device and bottled immediately.

Distillation:

Most essential oils are extracted using the distillation process. In this process, the plant part is placed on a grid that's inside a container called a still. The still is then sealed. Water, steam, or a water and steam combination swirl inside the sealed still and slowly break down the plant, releasing its volatile components and turn them into steam. These components then rise up and collect in the condenser. Once the condenser is cool, the components revert back to liquid form and are collected in a separate container. Once the essential oil separates from the water, it is siphoned off and stored.