There are three ways that are recommended. Lets look at each one separately.
Topically: This means we can apply the essential oil directly to our skin to either the affected area, bottom of the feet where we have the largest pores so that they are absorbed quickly, back of the neck, along the spine or on reflex points on the feet, hands or ears. Massage is also a good way to stimulate the muscles and tissue to help healing. In a warm bath and on a wet compress is also a great way to get benefits. Only 1 or 2 drops are necessary as essential oils are highly concentrated and some oils may need to be diluted prior to using.
When applying essential oils topically you may want to dilute and there are several carrier oils that work well. I like fractionated coconut oil as it applies easily and absorbs fast. Other oils such as virgin coconut oil, grapeseed oil, sweet almond oil, olive oil or any good food grade oil will work in a pinch. Please note that if you are using essential oils on children you should dilute until you know if your child is sensitive to that oil. At any sign of redness or burning, you need to dilute further or discontinue use.
I like to use this as a guild line when I make blends:
Therapeutic: 28 drops to 1/2 ounce
Massage: 50 drops to 4 ounces
Aromatically: This involves the direct inhalation of the oils or a vapor of the oil particles. Diffusion in a diffuser with a cool mist is best as heat will cause the oils to change and possibly not be as effective therapeutically. You can also just inhale directly from the bottle or apply a drop to a cloth or tissue. Essential oils can also be used as a perfume and still maintain their healing benefits.
I also like the benefit of using essential oils in my household cleaning such as Lemon in olive oil for polishing wood furniture, lavender in my laundry wash and on a rag in the dryer instead of a dryer sheet, I also disinfect with Melaleuca in a spray bottle just a few drops with water. Cleaning surfaces, lemon essential oil with vinegar and water mix in a spray bottle. Love this for cleaning my stainless steel appliances, windows and mirrors.
Internally: Please note that not all essential oils are safe to take internally. Be sure that your oils have a nutritional supplement label on the bottle prior to using internally. I personally use a couple drops in my water such as lemon or peppermint or any other beverage you like such as tea or almond milk. Essential oils can be used in cooking and added at the end of the cooking period. To add just use a toothpick to get some of the oil and add it to your food or try it a drop at a time to taste.
One of the best reference books on essential oils is the Modern Essentials, A Contemporary Guide to the Therapeutic Use of Essential Oils. Another book I have enjoyed is The Complete Book of Essential Oils and Aromatherapy by Valerie Worwood. Both of these books can be found on Amazon.com
The oils will work for everyone but not every oil will work the same for each person. You do have to find the oil that fits with what your body needs at that time.