Aromatherapy is a natural technique that uses the aroma, oleo essencial, and particles released by different essential oils to stimulate different parts of the brain, helping to: 
Relieve symptoms of anxiety, insomnia, depression, asthma, or cold;  
Promote well-being;
Strengthen the body's defenses.
Although natural products are used, it is important that aromatherapy is guided by a naturopath or other specialized professional, to find out which is the best essential oil to use in each case.
What is each essential oil for
Some of the essential oils most used in aromatherapy are:
Essential oil
What is it for

Rosemary
Mental tiredness, lack of memory, difficulty concentrating, headache, migraine, muscle pain, and joint pain.

Lavender
Excessive stress, headache, colds, insomnia, and breathing problems.

Lower leg
Physical or mental tiredness, dizziness, irritability, headache, lack of concentration, menstrual cramps, and difficulty in relaxing.

Jasmine
Decreased libido, breathing problems, excess stress, depression, and muscle tension.

Bergamot
Excessive stress, depression, anxiety, skin infections, poor digestion.

Chamomile
Excessive stress, muscle tension, depression, and inflammation of the urinary system.

Eucalyptus
Breathing problems, headache, migraine, muscle pain, fever, and muscle tension.

Lemon
Lack of concentration, anxiety, excess stress, lack of energy, weakened immune system, headache, poor digestion, and fever.

Sandalwood
Chest pain, excess stress, muscle tension, decreased libido.

Ilangue-Ilangue
Anxiety, excess stress, headache, nausea, high blood pressure, intestinal problems, or reduced hair growth.

These essential oils can be used separately or at the same time to alleviate several problems at the same time. However, mixtures must be indicated by the naturopath, as they must be used in specific formulas.
See also the most suitable aromatherapy oils to treat anxiety and to lose weight.
How to use oils
The main and most beneficial way to use essential oils is inhalation, however, they can be used in different ways, adapting to the problem to be treated or to the lifestyle of each person:
 
1. Inhalation
Inhalation is the most complete way to obtain the effects and benefits of essential oils, as it allows the molecules to easily reach the limbic system of the brain, creating changes in the functioning of the body, which make it able to heal itself.
To make inhalations, start with light inhalations and then increase the number of inhalations and the intensity, as indicated:
Short inhalations: 3 to 7 breaths in a row, several times a day;
Average inhalations: 10 to 15 breaths in a row, several times a day;
Long inhalations: 10 to 15 minutes of breaths in a row, 2 to 3 times a day.
To make the inhalations correctly, you should breathe the oil directly from the bottle, inhaling deeply and then holding the air for 2 to 3 seconds, before exhaling. 
Ideally, certified organic essential oils should always be used to avoid inhaling pesticides and other chemicals that can end up intoxicating the body.
2. Aromatizer
In this case, 2 or 3 drops of the chosen oil are added, inside a device with water that creates a cloud of smoke that releases the aroma throughout the room. 
A more economical solution to the use of the aromatizer is to place the drops in a cup of boiling water, for example, because as the water evaporates, the aroma is released into the air.
3. Evaporation
The evaporation is to apply a few drops on cotton balls, swabs, or a clean cloth, allowing the oil evaporating go and releasing its aroma.
This is a great way to regulate the intensity of the aroma because the closer you are to the cloth, the more intense the smell will be. This is also a good technique to use at work, as cotton, or cloth, can be placed in a cup on the table.
4. Sprays
The spray helps to spread the aroma to all the desired places, that it is enough to add a few drops of the essential oil in the spray tank and fill it with water. Before using the spray, shake the packaging to mix the oil again, avoiding spraying only water into the air.
This is a great way to purify the environment of a room in the house or even to use it in the bedroom of someone who is recovering from an illness, for example.
5. Vaporization
This technique should be used especially to treat respiratory problems or colds because, in addition to releasing the aroma directly to the respiratory system, it allows the inhalation of water vapor that hydrates and relaxes the airways.   
To do the vaporization, you must put boiling water in a basin and then add a few drops to the water. Finally, one should breathe in the smoke released and, if possible, cover the head with a towel to concentrate the water vapor. However, vaporization should not be used on children under 7 years of age. 
6. Massage
Massage is the perfect way to apply essential oils directly to the skin, in order to treat muscle pain, infections, skin problems, or joint pain. To do this, just mix a few drops of the desired essential oil in vegetable oil, such as rice, sesame, or coconut oil, for example.  
Ideally, in massage oil, only 1, 3, or 5 essential oils should be mixed, to ensure that there is no change in the molecules and are able to be absorbed by the skin.
7. Baths
Baths mix the benefits of vaporization, as they allow inhalation of water vapor and aroma, and the benefits of massage since they allow skin contact with oil. Thus, they can be used in almost all cases.
To make an aromatherapy bath, fill the bathtub with a little warm water and then add drops of oil until the desired aroma is obtained.