Even the most basic perfumes should blend three scents, or "notes." The combination of a base note, middle note and top note (added in that order) is what causes a perfume's aroma to subtly change the longer it stays on your skin.
The base note is added first and lingers the longest on your skin -- some essential oils can hang on for a couple of days. Base notes are often from the woodsy family, but vanilla, vetiver and patchouli are common, too. The base oil should make up about 20 percent of your blend.
The middle note makes up the core of your scent -- it should comprise around half of the oil you use. It will usually evaporate within two to four hours, leaving the base note to react with your skin.
The top note is what gives the first impression of your perfume -- you smell it immediately upon spraying. It's the last thing you add to the mix (about 30 percent) and the first scent to evaporate, usually within a couple of hours. Citrus and floral oils like orchid, chamomile and anise are popular top notes.
A few drops of a "bridge note" are sometimes added at the end to help the other notes blend together more smoothly. It's often lavender, vanilla or a very mildly scented "carrier" oil, like vitamin E or jojoba oil, that doesn't evaporate from the skin.
So you don't waste precious ingredients when you're playing around with scents, make samples on cotton swabs and let them sit overnight. If you're happy with the combo in the morning, you'll be ready to take the next step: making the actual perfume.