Essential oils all have therapeutic properties attributed to them but they don't necessarily have a fragrance that everyone likes. Firstly you have to decide whether you are using the oils solely for their healing properties, or to use them for an emotional response.
From a fragrance perspective, oils are categorised by having 'notes'. These are either 'high' 'middle', 'low', or a combination.
The high notes are the ones that first register with the brain, but they evaporate quickly. Middle notes are the 'meat' of the fragrance, they register behind the high notes, but they linger a lot longer. The low notes are the earthy, deep lasting fragrance that lingers a long time. Compare it with the perfumes you buy, the more expensive fragrances linger on the skin for a long time, you probably only need to apply them once a day. The cheap copy-cats may smell similar initially, but the fragrance is short lived. This is because the middle and low notes are by far, more expensive to produce in the perfumery industry and are used sparingly in the copy-cats.
A good fragrance should be a combination of notes that join together seamlessly, so that no one single scent is discernable. Fragrances have been likened to an orchestra, the combination should be pleasurable, but nothing stands out from the rest.
A good rule of thumb when combining oils is to first look at the response you want, seductive, uplifting, stimulating, relaxing etc, then choose the oils that satisfy these requirement and look at the notes they have. The bulk should be made up of middle notes, with a little of a low note to mellow the effect and 'round' it off, then choose a high note to compliment it.
There are no hard and fast rules, experimentation is the key. Make only very small quantities to start with, write down the recipe you have used, make any minor changes and record these too. When you find the combination you like, make up a larger batch.