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. Learn which oils match the benefit and aroma you're looking for.
Then experiment with quantities and combination's 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!