Begin on a five-tone scale starting on middle C. Play even quarter notes up and down in step-wise thirds: C-E-D-F-E-G-F-D-C. Hold the final C longer than a quarter note if you wish.
Use pure Italian vowels while repeating this exercise at middle C with two pitches per vowel. [i] for C-E, [e] for D-F, [a] for E-G, [o] for F-D, and [u] for the final C. First sing it with your natural vibrato at a mezzo forte dynamic. Sing with the most relaxed and open throat possible, without excess tension in your tongue or jaw. The tones should be easy and free. Remember this relaxed feeling, because singing with straighter, pure tones should feel the same. Repeat the exercise, singing the same way but leaving out the piano. When you arrive at the final C, check to see that you are still in tune. Pure tone singing must be done exactly at the center of each pitch, so make sure you can sing in tune with your vibrato before attempting to take it out.
Listen constantly for intonation. As a helpful guide, try to sing ascending intervals slightly bigger than you think they should be, and try to sing descending intervals slightly smaller than you think they should be. This will cause you to listen more and to carefully place each tone in the center of the pitch.
Repeat the exercise again with vibrato, but this time sing it at a mezzo piano dynamic. Allow the tones to be light and buoyant without pushing them out uncomfortably. It should feel like a sigh, with an open throat and a floaty tone.
Repeat again at mezzo piano, but this time sing at a slower tempo, checking with the piano at each pitch for the second half of the beat. Sing the C, then play the C to ensure you are precisely in tune. Move on to the E without the piano, then play the E on the second half of the beat. Run the exercise this way until you are certain that you are singing in tune. Run the exercise again without the piano to re-establish a legato line.
Do the exercise again without the piano but try to slow down your vibrato. Concentrate on the hooty sighing tone quality you established before and on moving the breath through the phrase. Repeat the exercise, playing the piano on the second half of every beat, to ensure that you are still in tune. Do this as many times as necessary until you can sing it precisely in tune with a slower vibrato.
Repeat the exercise and remove virtually all vibrato from your tone while keeping the sigh in your sound. Mentally insist on perfect intonation. Check yourself with the piano, then repeat without the piano.
Move up a half step to C#, once you have mastered this scale. Work all five-tone scales up and down your range. At the top of your range. Do not sing louder. Keep the hooty, sighing space open and alter your vowels. [i] modifies to [I], [e] modifies to [E] or [I]. Insist on perfect intonation with each pitch.
Keep in mind that all pitches on a piano are slightly out of tune because of our modern tuning system. If you are singing purely a cappella with a choir, the tuning will be slightly different than when you sing with a piano. Always sing in tune with the sounds around you. Tones that are in tune with each other ring in a way that slightly out of tune chords do not. Listen ahead through each phrase and make sure that you end in tune with the pitches around you every time. Pure tone singing has much to do with the sounds you make, but it has much more to do with what you do in response to what you hear, especially if you are singing in an ensemble.
Practice singing with pure tones every day. You are training vocal muscles to move in a certain way. Like any other muscles, they must be developed and nurtured. If you experience any pain during this process, you are singing incorrectly. Sing with a sighing, open throat and a floating tone and always try to make the most beautiful sounds you can. These sounds tend not to hurt, and thus they are the healthiest tones. Singing feels good when it is done healthily. Make vocal health your ultimate goal and you can succeed in singing in any style.