Try out this simple exercise. Pick three different materials that are known to have very distinct evaporation rates: one highly volatile, one that is known to stay around for a while but will eventually evaporate, and one that seems to go on forever. Label three different scent strips with each material and then apply them. Wait for a few minutes, then smell each one. Make notes about how they smell, which sounds the most dominant. Leave the strips, and try smelling each of them again later (a while later or the next day). Make more notes, and see what is still around and what the new scent is like.
Top notes are the most volatile of ingredients (think citrus, and fresher aromatics). They give the fragrance a sense of brightness and energy, along with an initial impression. Too many, or too strong, and the opening of a fragrance can be harsh or unconnected to what follows.
Heart notes are the middle characters. These materials last longer than top notes and are often made up of florals, spices, and other more green ingredients. As the highly volatile ingredients start to evaporate away, these materials can take the center stage. Too often the heart notes are viewed as coming into play after the top notes have completely disappeared, however they often blend with the top notes or dominate the opening of a fragrance.
Base notes are usually very slow moving, even tenacious materials (think woods, resins, ambers, musks). They provide depth and warmth, and give the fragrance its support system. Too much or strong concentration of these materials can swamp out subtler materials and overshadow or hide the heart notes.
It can be helpful, as you get going, to jot down notes about what the materials in your formula are doing during different stages of evaporation. You may be surprised at some of the materials that act differently than what you expect (diluting strong materials can help you get a better idea of what they are doing). The above description is very general and can be helpful for getting started. A scent blotter you kept from the initial application can help provide further development and drydown notes for a better understanding of the scent.
When we think of top, heart, and base notes we can begin to see patterns. Many beginning perfumers feel that they must balance their formulas so that there is equal parts of each type of note or so many of each type, however this need not be the case. The best way to utilize these materials is not in a prescribed fashion but more to realize the roles they can play in a formula.
When creating a fragrance, keep in mind what role you want each ingredient to play in the formula: Do you want this material to really help the fragrance open up? Do you want it to carry through as the character? Do you want this ingredient to help with structure, provide support for other materials, or last into the drydown? After you have created a formula that is well balanced but you feel like something is missing it is more helpful to revisit what your formula contains and tweak the amount of one of the materials and try comparing the original to the new one. Adding ingredients to try and balance the types of notes is much less helpful.