Singing into your flute is a technique that is most commonly used in jazz band. Jethro Tull uses this technique in the song "Bouree." In order to practice these techniques you need your flute, a place to practice and patience.
When you perform this technique it sounds like the tone and a buzzing noise. There are two different ways to get the tones. The first way is to make an O shaped opening on the lips and hum any one tone. Next take the flute up to your lips and the force of the air through the lips should be enough to make a sound. Make sure that the flute is touching your lips. You know you got it when you can hear the tone you're playing and the buzzing from your lips.
The other way to do it is to play a tone, and then add the humming. This is a little harder to do. Once you have the buzzing sound, experiment around. Try to hum lower and higher while keeping the tone the same. Try moving from note to note while your humming stays the same.