An essential oil blend is created for aesthetic and/or therapeutic benefits. No special, mysterious knowledge is required to produce a successful blend but a spirit of creativity and knowledge of oils will help, along with a clear sense of the intended purpose of the blend.
The Nature of a Good Blend

A good aromatherapy blend uses all-natural, pure essential oils and delivers a benefit. The oils in it should be synergistic to support and enhance each oil's effect.

Before beginning your blending work, it's important to understand essential oils in general, as well as the specific characteristics and aroma profiles of the oils you want to work with.
The Nature of Oils

Essential oils are volatile. When you uncap a bottle of oil and expose it to the air, its aroma profile immediately begins to develop and change. The three main stages of these changes are known as the top note stage, middle stage, and dryout stage. These stages of volatility coincide with three characteristics of the aroma profile known as top notes, middle notes and base notes. The top note stage takes place first and corresponds with the most volatile, unstable aroma compounds of the oil. The aromas of these compounds can be very subtle and fleeting, sometimes lasting for only a few seconds before they dissipate. As the top notes evaporate the oil enters its middle stage in which the body is most developed and balanced. Some of the top notes remain, the middle notes are most apparent and the base notes are beginning to develop.

As the middle stage progresses the base notes become more and more noticeable because they consist of compounds that are slower to volatilize. When the base notes finally evaporate, a faint dryout note remains. The aroma of the dryout stage is thin, sometimes powdery and bittersweet, and suggests the presence of the oil. It can be thought of as the matrix or skeleton of the oil.

Depending on their individual aroma characteristics, oils themselves can be categorized into top notes, middle notes and bottom notes. A very delicate, highly volatile oil like bergamot is considered a top note oil. A well balanced, somewhat tenacious oil like clary sage would be considered a middle note oil. A deep, rich, very tenacious oil like patchouli is considered a base note oil.

When creating a blend, strive for balance between top, middle and base note oils. A well balanced oil has a complex aroma profile and maintains the aromatherapy benefit throughout the various stages, although the characteristics of the scent may change.
How to Learn About Essential Oils

Before you begin blending, analyze the various aroma profiles of the oils you want to work with by dispensing a drop of oil on blotter paper or unscented tissue. Concentrate, inhale and note your impressions. Build a file of notes for future reference. Trust your impressions and experience of the oils. What do the aromas remind you of? How do they change and develop throughout the various stages? How does the oil's fragrance make you feel?

As you develop a descriptive picture of the oils, think about how they'll work with each other in a blend. What part of the oil do you want to work with? What effects do you want to enhance? What characteristics do you want to tone down?
Relaxing Essential Oil Blend

Imagine an "End of the Day Blend" to help you relax and unwind after a busy, stress-filled day. This blend should be soothing and relaxing, but also uplifting to chase away the cares and worries of the day and give you a little boost to carry you through an evening at home.

Two excellent relaxing oils are chamomile and lavender. Uplifting oils include bergamot and lemon. A blend of these oils would produce a light, sweet aroma with complex top and middle notes. The bergamot, lemon and chamomile would give this blend a fruity sweetness. The lavender would make the sweetness a bit more floral-herbaceous. If you want the blend to be a little less sweet and more stimulating, a small amount of herbaceous, penetrating rosemary could be added. The relaxing, spicy-woody oil of rosewood might serve as an effective base to this blend.
Common Sense Blending

Learn which oils match the benefit and aroma you're looking for. Then experiment with quantities and combinations until you create the blend you envision. Always keep notes on what you're doing so when you hit upon that perfect blend you can repeat your success. Happy blending!