Measure the height of the nose (from top to bottom) and with width (from one nostril to the other). That'll help you determine the shape and form of the nose really quickly. However, just having these two measurements are not good enough. The drawing is still just two dimensional.
To make it look real, pay a little attention to the ball of the nose. You can't really use lines to bring out the ball of the nose. So you'll have to rely on your shading skills. Shading is always useful for bringing out three dimensional effects.
Once you get the tones of the shades right, the nose should come out really nicely. Try drawing just noses for practice, then move on to the other features of the face.
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