There are two parts to the concept of mixing essential oils.
The first is the mixing of the oil in a good carrier oil to better disperse it on the skin when massaged and to dilute the stronger ones so they don’t cause irritation. The second concept is mixing different essential oils together to create either a stronger medicinal effect, or a nicer perfume, as the case may be.
A carrier oil is a base oil used in massage. Essential oils by themselves are far too concentrated to rub all over the body, not to mention that the cost would be prohibitive. So when massaging the oils the therapist will dilute the essential oil in a carrier. This will usually be something like sweet almond oil, safflower or sunflower, olive oil or grape seed. Most massage therapists have a preference.
Sometimes different oils are used as carriers for use on the face. Jojoba oil is great for this as it is not greasy and is absorbed well. Also good for the face is rosehip seed oil, which is very quickly absorbed but has added benefits to the skin such as vitamins and minerals etc. Apricot kernel oil and avocado are other oils that, as well as being good carriers for the essential oil of choice, have their own added benefits to the skin.
When diluting an essential oil in a carrier a ratio is normally used. Something along the lines of ten to fifteen drops of EO per ounce of CO, less for the delicate facial skin. Its good to experiment and see which carriers and ratios will work best for you.
There are many ways to blend the actual essential oils together for a desired purpose. What I usually do is create the essential oil blend first, and then using the desired ratio mix the mixture into the carrier. This creates a nice smooth blend of all the elements. Sometimes letting the mixture sit for a while brings out all the lovely notes in the aroma making for an even nicer experience. We mustn’t forget that it’s called “aroma-therapy” and that the scent is a part of the medicine too.
There are many ways to blend the actual essential oils together for good results and a lot of it is very subjective. You will have to use trial and error and let your nose lead you. But there are some rules, or at least guidelines. In regard to the perfume, oils from the same category blend well together, for example peppermint and spearmint, both from the mint category, or nutmeg and cinnamon, both spicy.
But some categories blend nicely with others. For example orange is lovely with cinnamon, which is citrus and spice together. Medicinally there are some different rules, for example blending pine, eucalyptus and rosemary for shortness of breath. Have some fun with it and experiment a bit, but always respect essential oils, they can be powerful allies.