What Is Aromatherapy
Aromatherapy is the use of fragrant oils extracted from plants in the treatment or prevention of disease, often called essential oils.  The term essential indicates that the oil carries distinctive scent (essence) of the plant, not that it is an especially important or fundamental source.

Since essential oils are so potent, many can irritate the skin unless diluted with carrier oil such as sweet almond oil or olive oil.  Essential oils are extracted by distillation but can also be extracted by expression or solvent extraction.

How To Use
Essential oils can be used through the skin (massage, shower, adding to bath water), and inhalation (ceramic burner, vaporizer, handkerchief, pillow).

Inhalation.  Add a few drops into a ceramic burner with a lit candle underneath.  When the oil warms up the scent will be dispersed into the air.
Vaporizer.  Add a few drops to boiling water and inhale the vapours.
Skin Application.  Add several drops to almond or olive oil, pour into your bath water, climb in and relax.  You can also buy ready made oils for baths from some pharmacies.  If you shower, mix up half the quantity and use on your shower sponge, flannel or body scrub.
Infusion.  Some herbs and essential oils can be used as an infusion in a tea but always check before use.

Which to use
Firstly, you need to identify which essential oils are best for treating stress and meeting your needs.  Lavender, clary sage, chamomile and lemon are a few of the common ones.

Lavender is a relaxant
*add a few drops to warm bath water, climb in and relax.
*add a few drops to water and put into a spray bottle.  Spray around your home when you feel stressful.