Assess the nature of the vocal tone you generally produce while singing. Consider whether it is thin and weak or rich and powerful. Is it laser-focused and piercing or broad and warm-sounding? Perhaps it's breathy, without much support. At times, you may prefer different tonal variations; but, you'll never want a weak or unsupported tone, nor will you want a tone that lacks proper vocal resonance.
Work on correcting any of the negative issues mentioned in Step 1, which prevent the nasal cavity from doing its job of producing a pleasing vocal tone that expresses the natural beauty of the female voice. You can correct a weak and thin or otherwise unsupported tone by developing good air flow, through proper use of your diaphragm.
Experiment with focusing your vocal tone in various parts of the resonating chamber that consists of your throat, nasal cavity and sinuses. Begin by forcing the tone out through your nose and sinuses to see what that feels and sounds like. Next, try opening your throat and centering the sound in the larger resonating space you've just created, and note the way this sounds and feels. You'll find that the former method creates a tone that does little to complement the female singer's natural voice, while the latter creates a richer, fuller, more compelling sound.
Practice singing in the more centered tone mentioned in the previous step, being certain to keep the throat open and note the resonance that indicates correct use of all the resonating spaces together. Before trying for a more individualized tone, you need to master keeping your tone consistently centered.
Adapt your vocal tone to the sound and style you prefer, once you've mastered proper tonal placement. This is where the female vocalist can really begin highlighting the uniqueness of her own voice. For a more brilliant, focused tone, increase air flow, directing it up through the nasal cavity. At the same time, keep your vocal tone centered. For a richer, warmer, sound, widen the resonating space at the back of your throat, using sufficient support from the diaphragm to create a resonant tone without specifically directing the sound up through the nasal cavity.
Continue practicing and experimenting to discover and develop your own personal singing style. Be sure to maintain good support from the diaphragm at all times. As you practice, remember to focus on the goal of achieving nasal resonance, rather than nasal tone, which will lend greater beauty to your voice. When you've done this, you'll have learned the proper use of the nasal cavity in female voice training.